The problem is cutting a large log with a short bar that doesn’t reach all the way through.
Example: Say a cut curves to the right a couple inches as you cut through the log, this is doubled when you then go to the other side of the log and are facing the opposite direction. Now that side’s cut will miss the other side by 4” at the bottom.
As others have said, try to use as much of the cut you’ve made as a guide. Yet, sometimes the cuts still don’t meet.
This is where wedges and a hammer are handy to pop the tree apart when cuts didn’t meet.
Check that your saw is cutting straight. Uneven sharpening and/or a bar that is worn uneven on one side (left or right rail) can make cuts curve. Do you unconsciously push the saw so it curves?
Try to cut as much of the log with the saw facing the same direction instead of going to the other side of the log where saw faces opposite direction increasing the chances cuts don’t meet. Try to cut all of log, except what is touching ground, from one side of log. Cut far side from top with saw nearly vertical at times and work back towards yourself.
My 2 cents.