Some photos would be really helpful.
However the usual suspects are
1) worn , broken or bent blades
2) mower engine not running at full speed
3) mowing too fast
4) wrong blade for the type & height of grass being cut
5) trying to take too big a cut
6) blades on upside down ( yes ever pros do this on occasions )
7) slipping deck belt
8) junk caught under the blade
9) bad bearings in spindle ( usually from owners who use the wash out port )