There could be several reasons why. Some slide systems use rollers at the cantilever end, some use nylon slide shoes, etc etc. Can you post a picture of the underside of your slide with it all the way out? This will help determine which slide unit you have and what could possibly be wrong.
I assume that the original floor was carpeted in the slide area ... and it has been replaced by a wooden floor.
If that is the case ... look under the slide to see if there is a "Teflon" glide ... or if it has been replaced by a "carpet" glide ...
If you have the Teflon glide then you have two choices ... replace it with a carpet glide (available from Winnebago ... or you can make your own) .... or there are some strips that you can put on your floor each time you bring your slides in that will protect your floor ...
if you have a carpet glide then you probably need to have your slide adjusted ... I think there are directions for doing that in the service tips on the Winnebago website ...
We replaced the carpet with vinyl tile in the past year ... that required that we replace the Teflon glide with a carpet glide ...