I'm running a mobile campaign on PopAds.
I tested several bids when I started the campaign and I found my winning bid.
The campaign ran successfully with this bid for a couple of weeks then now the traffic I'm getting is very low.
What should I do now? Should I start testing new biding points with a high bracket that my previous test? or does that mean that I'm doomed?
Can't set it and forget it with pops. Test higher bids.
makeitrain has just nailed it in the head there! 
The bidding landscape is changing constantly with competitors coming in and going out and changing their bids and targeting and blacklisting/whitelisting etc. etc. So the optimal bid for today will be different from the optimal bid for tomorrow.
It sounds like you may have gotten outbid. Try increasing your bid to see what happens. I don't know if you've tested really high bids before, but if you haven't, it may be worth it to bid top position and just set a low campaign budget just in case. On popads (and some of the other sources) when you bid higher you can get better traffic. Sometimes by bidding sky-high you'll see good ROI from the better conversion rates. Always worth a test, especially since your camp is already profitable.
Amy
I have the same experience with popads. For a week or 2 everything will be smooth sailing then all of a sudden I go from thousands in traffic to few measly hundreds. Increasing my bid usually doesn't help much since someone usually outbids me by some absurd price like $80 cpm. All though I'm sure they're not paying that much, increasing my bid yields very small increases at a higher cost. I'll usually pause the campaign for a few weeks and try it again with better results, but the bidding war seems to just happen all over again.
Ultimately, the most effective thing to do would be to improve your funnel - mainly by testing more offers. The higher your margins are, the more you can afford to spend and still remain profitable.
Segregating your camps to target different traffic segments and finding offers that convert best for each segment is another good method. For example you have one camp for Android and another one for IOS. Or you can have one camp for carrier traffic and another for wifi traffic. That way, not only will you be able to find the best-converting offer for each segment, but you'll also be able to bid appropriately for each segment. Only do this for major traffic segments that are giving good traffic volume and conversion rates though.
And of course if you cut placements that don't convert well, that will allow you to pay more as well for placements that do convert well.
But yeah - sometimes you just can't compete with the crazy bids. The two most-common "strategies" (if you'd even call them that) are 1)just pause the camp for a bit and restart like spacey said, and 2)bid even higher than your competition and run at a loss and hope they'll back down first. Maybe the pros will chime in with tips on how to win bidding wars. Personally I just do the pause-and-restart thing, and also test more offers. I prefer to test more geos and verticals and traffic sources, instead of getting hung up on bidding wars - but that's just me. 
Amy
Thanks a lot for the very insightful feedback.
Although it seems that the campaign strated getting back on track since yesterday, here is what I will do moving forward based on the feedback I got here:
A quick optimization process:
1- I will do another round of bid point testing whenever I get outbid.
2- I will keep looking for pockets of profit in carrier, wifi, borwsers, OS versions...etc focusing only on pockets that have good volume.
3- As usual keep blacklisting and whitlisting placements whenever possible as well as constantly testing new LPs
4- Ask for a pay bump if possible.
Otherwise, if all the above fails I will either pause and restart later when the average bud goes down, or keep running at a loss at a very high bid in hopes that THEY would give up.
Sorry if I'm repeating stuff I just want to recap for future reference and for everyone's benefit.
Thank you,
Ahmed
Nice summary! I would make the following changes:
-I'm leaning more and more towards avoiding whitelisting when it comes to pop traffic, because most of the time it will just leave you with small traffic volumes, and performance will often decline soon after. Much better to improve the funnel and then blacklist placements.
-Try to focus on testing offers instead of LPs. I mean certainly you need a good LP for the whole campaign to work, but generally speaking, you can achieve higher increases on ROI by testing more offers (once you find a decent LP).
Amy
Thanks Amy.
Yes you're right I had the same experience with whitelisting as well, in addition to some placements being taken of the network as well. It was a routine in the process though, but I will give it less priority from now on since you pointed out what I experienced as well.