Make sure the handles are all the way out - either one pulled in just a bit will also kill it. Handles out, release brake, then pull handles in.
If it isn't a problem with the order of brake release/handle use, then it's likely the seat switch. I have a problem with mine every so often, and it about drives me nuts. Sometimes I can just bounce on the seat a bit, but usually I have to get up, lift the seat, pull the switch connector off and on a couple times to re-seat and that does it.
The connectors don't look corroded or anything, but re-seating seems to do it. I'll have to clean them real good sometime...