blockchain says Not enough funds i have 30-40 $ in my account

now i have pay for something but when i add 20$ amount i say Not enough funds.

and when i click on maximum it fill the field by 4.15$

what is the problem and how can i send money !?
as @mcccs mentioned i changed the network fee to a custom value and it show more fund to transfer

when i set the fee to max (137) the transferable fund is 4.15$