Gear issue on yesterdays ride.

Well i ride my mountain bike in the winter, i think its better for the conditions, but my commute only really has one section of downhill for me to change out of the middle cog, normally i dont bother as there is no point, but yesterday i did, it felt a bit stiff to change up but it did do, then i got to the bottom of the hill tried to change back to the middle cog and it just wouldnt change! was set stiff on that top cog, so i had to do the rest of my commute there and my commute back all on the top cog, was abit harder than normal but it didnt take me any longer.

Took it to the lbs so they could take it apart to have a look, and the cores? a build up of crap from the road that had got stuck into it!

told me i should keep changing into top and bottom cog more often