And there is the reason for your problem.
Basically, you will have a Yseam at EACH of the 6 points on a hexagon (the tumbling block) when it connects to ANOTHER tumbling block.
So if you sewed all the way to the end on those 2 seams, there is no room for the Y seam, which is why you have a bunching up when you're trying to join blocks.
So, take two other tumbling blocks (that you haven't joined to anything yet). Very carefully, pick out the stitches back to where you would have stopped if you knew ;-) you had to be preparing for a Yseam to come later. Do this on all 3 seams on those 2 Tumbling Blocks.
Actually, take a bit more, then resew the seam stopping at the correct point. Do you mark yours or have you been eyeballing it? I mark mine.
Then try joining a couple of those blocks. You'll see how much easier it goes together.
Go. Sew. Report back. ;-) ;-)