recently is very difficult to push

I like this vacuum, I've had it for a few years. Just this week it became very hard to propel. The model # is 08817  plant s, manufacturer's code 003A  My hsuband called ;your 800 # and was told to slightly pull the handle back for 45 seconds,  After doing this it was still hard to push on the carpet. Any other ideas?  Like should it be running when the handle is pulled?