There are generally ever two reasons why any adhesion doesn't work.
Either A, it's a lack of friction or B, not smooth enough area which makes it difficult to stick.
Now pray paint is a really bad idea due to it's liquid nature as it'll just peel the sticker away.
So yes, I'd too agree with the mod podge or somehow increase the friction coefficient of your armor to allow it to stick better. But if this doesn't work, you can just try to glue it on.
Well I hope this helped.
When in doubt, just take the next small step.
With that next step take a deep breath. It calms the mind.
When the mind is calm you realize the best is still yet to come.