I've been having this problem too so I took blocks apart and sewed again with smaller seams and then I read where a lady had this problem and she pressed her seams open instead of to the side and said that flattened things out and made the block just the amount she needed. I've also just found this which I found interesting.
https://www.youtube.com/watch?v=c0Q-cl4OmlE and also this
https://www.youtube.com/watch?v=RsfgFzm55Iw