Small Engines (Lawn Mowers, etc.): self propelled feature, belt tension, push mower

Question
i have a troy bilt self propelled 21 in mulch push mower and the self propelled feature isnt working properly . i have replaced the front wheel to see if that would help but didnt when you tilt the lawnmower up off ground wheel turn great but as soon as they hit the ground it is like ther isnt even power

Answer
 Hello Michael:

 There are Several Possible Causes for this Problem. Send me the Model Numbers Off of the Mower, Please. With them I can Look at the Motion Drive System your Mower has and Can Better Assist you. Have you Checked the Belt Tension?Thanks.

 Respectfully

 John