Sometimes, when I upload panoramas, there are one or a few pictures that won't upload properly for an unknown reason. it will switch the picture for an error message at the upload screen, but if you don't look at it it will proceed to the stitching page without any other notice. when this happens, the panorama will fail to stitch. What I do in these cases, I change the name of the bugged pictures and then it works properly. I would guess having a higher number of pictures will increase the chance of getting bugged pictures, but if you look at the upload screen every now and then you should be able to see if there are pictures with an error message or if one picture is not uploading (it will idle for about 10 minutes without making any progress before replacing the picture for an error message so if your internet is not too slow, you should be able to notice this stop in progress very easily if you take a look at the upload screen every few minutes). I have no idea what causes this, and the bugged pictures work perfectly on other websites or in the computer, but just change their same to something else and everything will be fine (at least it works for me)
good luck