I take it you have either 2 or 4 blades fitted - whichever it is designed for. Have you tried reducing the forward speed of the mower at the same engine speed (i.e.,. using a lower gear)? If that fixes it, you could look at fitting more blades as a way to make a higher forward speed feasible. However if that doesn't make any difference, you have a cutting geometry problem - perhaps pushing grass down instead of lifting it up for example, due to fitting the blades incorrectly or having unsuitable blades.
