Teespring CPA disaster

Hi guys, I need help and critics on my CPA method.
So I started doing teespring ads now 2 months and I had 3 successful campaigns( sold 30, 50, 600 shirts) but lately it's not working so good for me.
In one huge successful campaign, I made OCPM ads with facebook. I started with $35 during 4hours as a test and when I got 5 conversions and a roi near 200% in this short period of time, I did another 4 hours ads and it worked.
The next day I spent another $150 during 12hours of the day and got enough profits and so it went up each day until I knew the niche is saturated and the profits started to drop. Results are around $5000 of pure profit.

Recently I wanted to have the same success with some campaigns but it didn't work. I'd have enough profit with the first $20 in 4hours of the day then I just scale up to $80 for the next 4 hours of the day and conversions will really drop.
Like with this campaign, I put $20 on FB OCPM ads, spent during 4hours, and get a ROI of 150% and 5 conversion then I decide to add $65 to my campaign and conversions dropped so badly so I stopped the campaign and decided to run it again the next day. In the next day I did put more budget but my roi just dropped near 10%. I just want to know what I'm doing wrong? How can I sell 5 t-shirts with $20 of ads then only $15 when I add $150 in budget?
This happened with alot of my campaigns where I'd see them as winners then I put more budget and it just drop so badly that I have to stop it.
Do I scale up in budget so fast? What about my method of doing ads just for 4hours on a $35 budget? If it's a wrong method then why it worked with one of my t-shirts and I sold over 600 tees with this same method?
Do I have to switch from tees to hoodies because summer is over?

I ask alot of questions but I'm new into making ads and I can't find what's really wrong with my campaigns and how I can't find a winner lately. Let's say there's nothing wrong with my designs as I'm a designer.

I'll really appreciate your help. Thanks guys