Hi,
I had a heavy Al patio door and had the same problem. There were two
single wheel rollers which had become twisted (with the weight). I
looked up 'Aluminium' in the yellow pages and sourced some spare patio
door rollers (I think it was a company called Alcon). Apparently all
new patio door rollers contain two wheels to cope with the weight.
Replacing the rollers was far easier than I'd thought. Get someone to
help lift the door off the rail. You'll see some holes at the sides
towards the bottom. In these are the screws for removing the bottom
part of the frame - I needed to remove these to replace the rollers.
I'd no problem putting the part back after replacing the rollers (I
didn't even need to drill any extra/new holes for the new roller).
Rory