My QB has 8 passing, 7 vision and 20 intelligence.

His natural position is Offensive lineman.

He is having 12-15 atempts per game and completes only 1 or 2 passes.

All his receivers have good speed, agility and catching ranging from 6-8.

Yes, you got the problem... last game 12 atempts and 7 were under pressure...

But my OL is very good, all of them have at least 13 str, 5 blocking and 5 footwork

5 blocking is only 25% of the total skill, and while a good start is nothing that couldn't be pulled from the youth academy. some people have blockers that are rated 12's already. which of course means there are d linemen with those kind of ratings give it time and your protection should increase.