I have a Ryobi trimmer, less than one year old, that was running great until about 2 weeks ago. I have checked the plug, good spark and gap. Seems to be getting fuel, will not start or even sound like it wants to turn over in any position (full, half or no choke). Help?!!?

This mower wouldn't roll backwards, forward was fine. It's a 3 speed and it lacked power on a steep hill. When it wouldn't roll backwards, I took off the wheels and gears, and the shaft would then roll forwards and backwards and I could hear the transmission gears turning. I cleaned and lubed everything, and reassembled. After reassembly, everything worked correctly, except it didn't move quickly up a steep slope. After about 20 minutes of cutting, the rear wheels started having problems again rolling backwards. I finished in about an hour and the rear wheels wouldn't budge backwards. Any advise would be appreciated.