When you say won't engage, what is it doing? Is it just spinning away? Is it grinding? Is it trying to engage? Is it just going clunk, and jamming? Is it going clunk, and just stopping?
This will help diagnose the problem.
The ring gear on the flywheel, these have been known to move on the flywheel this can cause similar issues. I think some of the reproduction ones are too loose a fit, and don't stay put.
Martin.