I have the same problim when I pick up repair jobs with differant brand mowers. I find I have to also use the 2X4 or 4X4 under the back edge of the ramp. I am going to keep a eye on this thread maybe for a better answer ?
I was having this issue also, what happened to me was over time my deck was becoming lower and lower, the reason was that right where the deck is welded and connects to the piece that lowers and rises the deck was getting worn, ultimately the weld broke and I needed to have it rewelded. If thats not the problem, you may need to re adjust the deck height by lowering the deck all the way to the ground to the 1st setting #1 and adjust the deck height screws. Good luck.