Skipped stitches. I have been having problems with skipped stitches. When I first got the machine

Question

I have been having problems with skipped stitches. When I first got the machine everything worked fine. But on the last few quilts I can't prevent skipped stitches. I have changed needles, thread, tension, cleaned, you name it I have probable tried it. I think my problem is timing. How can you adjust this?? Thanks