Question about completing an incomplete file

I tried to find a question addressing this particular subject, but didn't find anything, so here goes;


There have been a few files that I can start, and have download at a pretty good speed for awhile, and then it quits. Any attempt to restart it is in vain.

Then maybe a day or two later, I can find the same file by the same user, and if I click on it and start it from zero, it will almost always start again, download for a while, then stop again.

But I can never get this same file from the same user to pick up on one of the incomplete files and resume from the point it left off. Its very vexing, since the combined amount of data from the incompletes could easily have equaled a complete file.

So why can I start the same file from the same user from scratch, but not resume an incomplete?