Jack,
What you are saying is really quite a mystery. If you have it imported, and you can see it in the box, then it should print. Dumb question. You can import it and make the file, but it won't stay if you don't Save it. When, you right click on a name in the Patient List and choose Print Notes and Consultant Letters and then click on Compose Letter without printing, can you see it in the signature box?
OK, so if I had to bet, I would say there were two possibilities, one more likely than the other. The less likely possibility is that you need to reinstall, maybe only on one computer (to see if it will work).
The second is I am guessing you have more than a few users. You, nurses, receptionists, etc. When you import it, do you have your name in the Edit User List? So when you import the signature, it is always associated with you?