Trouble Inserting Axles

Sometimes they can be stubborn. Jiggle and twist on one side with the other blocked so it can't turn. See if you can slide it in any farther. Then switch sides and do it again. Back and forth, keep at it. You're trying to align splines here.
In some ways it can be like seating a torque convertor; jiggle/twist/push, jiggle/twist/push. It'll go eventually.