The tyres probably need wearing a little, I find that (even if they are older tyres) after a bit of bouncing they grip a lot more.
Also, I find that theres two ways to end up out of getting up (the way with one leg in front of the other right?) I find i can push myself so i stand up on my back leg or my front leg, changing so you push with your front leg might help, just an idea :L
Hopefully the tyres will wear in though