Hi, My Troy-Bilt Self Propelled Lawnmower, model #12AEA29L011, seems to me like it's not what it used to be. The mower is 10.5 years old. Engine seems to be in great shape. I have a very thick yard, (carpet grass). When I am cutting my yard and making a 90 degree turn I lift the front end of the lower and then after I turn, I lower the front end down and notice that at times the wheels won't spin. I had never noticed this before. On thinner grass I haven't noticed this issue before. Any comments on what maybe wrong are appreciated. Thank you