Facebook Threshold Automatic Droped

I facing a sweet problem.
at first, I tell briefly, I am using BRL Currency.
When I boost my Facebook post. I getting 75 or 30 BRL Threshold.
But after some time it drops automatic 7 BRL.
Why I am facing this problem?
if I get 7 brl normally. i have no problem.
but at first, I get 75 brl then it drops to 7 brl.

These are the automatic rules of facebook

Hi, is there a way around it. I usually get $25 threshold and they are automatically reducing it to $2 after some time.