2 months ago I purchased a new ExMark 32 in. Metro Walk behind. 1 month ago I purchased a sulky for it. At first it would pull me around in any gear, although I almost always used 3rd. gear because the grass is very thin and easy to cut. Now it will only pull me in 1st. and 2nd. gear. The belt is not slipping, the drive pulleys just stop turning. Is it normal for the transmission to have these problems so soon? Is there anything I can do, like a stronger transmission?? Thank you in advance for your help.