blower motor doesn't work all the time what could be the problem?

As Ed said. If its the fan switch. its possible you may get lucky and that by moving it through the full range repeatedly you may burnish the switch contacts enough to improve the connection. (This also applies to with manually controled rotary radio controls, and instrument dimming switches switches.) If that doesn't work, try to pin down if its related to if the vehicle has been sitting overnight and the body and engine are stone cold. A loose/poor connection would tend to be a tiny bit tighter then. If it CONSISTANTLY works better when its raining and doesn't when its hot and dry-a bad ground to body or chassis connection is more likely.