I have a similar problem with my hot plate/stirrer. The plate warms up considerably, but I cannot bring 100 ml of acetone in a beaker to a decent boil.
When looking closer, the plate us bulging upwards in the middle, which makes the contact surface between it and whatever is on it very slight.
I don't know why this has happened, and I don't have any good solution to the problem, but I also would be very greatful for any advice. I feel like using a sledge hammer to get it straight again, but the most reasonable option I guess is to get it repaired by a professional.
A temporary thing that might work however, is folding and pressing together a couple of meters of aluminum foil, and put it between the plate and whatever you're trying to heat up. It should increase the contact surface some, probably not enough though.