Those pesky hst. They get a fair number of people.
Check the size of them before putting the rest of the block together. They should be 4 1/2" each. If not it is a matter of seeing where it is that you are loosing out.
Cutting. Even is you slant the cutter in towards the ruler a bit you can end up with a slightly smaller piece. It adds up even though it doesn't look much different.
Sewing. Double check your seam. Especially with so many even one or two threads yet again add up
Pressing. Yes you can even loose some in the way you press. If everything else is good when you double checked. Try different ways. Yes even pressing open.