Hi Jim
I had a similar problem recently and nearly drove myself mad because it was a set of wedding pictures. The pictures were in the folder, but LR just would not recognise that they were. The problem turned out to be that after editing the pictures initially, I had then renamed the final edited set in Lightroom, using the surname of the customers in the filename, as is my normal practise. However, at some later date I had realised that I had mis-spelled the name with one letter wrong. For some mad reason I opened the folder in Bridge and renamed the files using the batch rename in that programme. Of course the files then had a new name and so LR would not recognise them! The names looked so similar that I just did not twig for quite some time. The problem with these programmes are that they are just too logical. Why the people at Adobe cannot come up with something like "do you actually mean you are looking for *****?", I don't know.
Anyway, I hope your problem is as simple. The filenames have to match.
Good luck
Jim