triple chain ring??

If your rear cog is in the smallest gear maybe you need to shift your rear deraillure to a lower gear (toward the center of the set of cogs) then shift into the middle chain ring. This might help.

Another thing is don't wait till you start the climb to shift. drop into your middle chain ring before you hit the bottom of the hill. You might spin like crazy but you will b more or less ready for the 20% grade.

Lastly, do have your drive train checked by a good mechanic, IF YOU ARE STILL HAVING A PROBLEM.

Smilingcat