Not sure what network you are using, but generally speaking:
... with CPA bidding you're in the hands of the algo so to speak. It's gonna send the traffic to the campaigns that make the most money for the network, so my bet would be that there are some competitors targeting the same traffic but their offers are working better or they are bidding higher and the algo rewards them with the traffic.
You can try to increase the CPA you're willing to pay to see whether the algo starts to favor your campaign more.
Another option is that the algo has a preset timeframe to evaluate the campaign and it's over now and your performance was not strong enough so the clicks dropped. But these period are usually not that long.
And also doublecheck with your AM regarding the traffic availability, it's also possible that they lost some strong pubs in the GEO (target group) that you are after.
Not sure what network you are using, but generally speaking:
... with CPA bidding you're in the hands of the algo so to speak. It's gonna send the traffic to the campaigns that make the most money for the network, so my bet would be that there are some competitors targeting the same traffic but their offers are working better or they are bidding higher and the algo rewards them with the traffic.
You can try to increase the CPA you're willing to pay to see whether the algo starts to favor your campaign more.
Another option is that the algo has a preset timeframe to evaluate the campaign and it's over now and your performance was not strong enough so the clicks dropped. But these period are usually not that long.
And also doublecheck with your AM regarding the traffic availability, it's also possible that they lost some strong pubs in the GEO (target group) that you are after.
Thanks for the advice.
The original campaign probably performed well from the start so the algo was favoring you, then it either lost steam a bit or someone else took the traffic with an even better campaign. It can take a while to get more attention from the algo again, maybe let it run for a while to see what happens.
This is exactly why I always prefer CPM bidding, no guessing about the delivery and why it's low 
Exactly what I will do.
I only use CPA as for some reason I have never got a CPM campaign to work.
If I take a whitelist and bid on CPM, I get no conversions, same impression amount and spend on CPA I get conversions
But that said I am not going to give up, I just took a list of all the sources with ah conversion rate of over 5%, made a whitelist and set it up for CPM bidding (started with the network average bid)
I just think that I need to learn CPM bidding more and the nuances of it, any tips and pointers happy to hear 
So the first piece of advice with CPM would be to take it slowly and do not try to bid very high straight away. You need to aggressively cut the non performing zones/placements.
I wanted to add to this thread and get some idea about volumes that people are experiencing.
Currently in the US i am able to buy between 100K-500K impressions per day from one traffic source (the average is closer to 100K. What are the volumes that I should expect there, (source is propellerads). Their inventory says 2.2 mil available. is the amount that I am buying high, medium or low? I feel like it should be more?
what have others experienced - thanks
500k out of 2.2mil available is already on the high side, 100k is not and you should be able to get more. Keep in mind you're not the only one competing for the traffic so the amount you're able to buy on any given day will be significantly affected by what your competitors are doing, how many campaigns they launch, how high they bid etc ... so there will be quite big fluctuations from one day to the next.