I'm running a mobile CPC campaign and the daily spend always remains about the same, but the conversions sure don't.

I launched the campaigns on Oct 28th, Nov 7th and Nov 24th. *Spend is about the same all days*. ROI is great 1st day of campaigns. I'll even get excited, up the CPC, and send in more budget. Bu after that first day the conversions slowly die off.
2 possible things about why this could be:
1. The first day the offer is new and there's interest. After that it gets less interesting/less conversions - (but there's still clicks?)
2. The platform is placing the campaigns in different spots the first day, then after that basically de-optimizes the campaign and kills my conversion while still taking spend.
Do you guys see this ever? Any other ideas?
All of the above?
It's hard to gauge what's going on if you launch/relaunch each week.
What traffic source? Did ad CTR change much? Or other metrics at the traffic source end? Did upping the bids bring in new inventory that performed poorly?
Im having a similar problem with a campaign I started running at the beginning of October.
I optimized it to the point of profitability at the beginning of November. It performed well for the first 2 weeks of November with a ROI of 80-100% and profit of 50-150$/day with some 0 days.
Now I'm getting mostly 0 conversions with the same amount of clicks as always, from the same publishers. The offer has a payout of 15$ and is a mobile games subscription offer.
I was direct linking it, so I tested with a simple "download game" style landing page over the past week and different ad copies, which ended up breaking even.
Im not sure how to go about this, should I scrap the current ad copies and try completely new ones and go back to direct linking?
I also havent been using any special angles, just simple "Play Now" banners, should I try to come up with a new approach?