Thread Stuck and Won't Come Up Through The Plate?

I've taken the plate off my Janome MC6600 machine, removed the bobbin and bobbin case, gotten the thread unstuck, gotten all the lint out and put it back together. I've rethreaded the machine and replaced the bobbin. When I push the needle down/needle up, the bobbin thread comes up a bit, but the top thread gets stuck on something in the machine. I've taken it apart several times and put it back together, but every time I do the needle down/needle up, the thread gets stuck.