Why are my dimmer switches malfunctioning?
Sounds like a simple case of a bad switch.

Note: a squirt of WD-40 into the right place in the sliding mechanism can help, or even fix the problem at least temporarily by lubricating the internal sliding electrical contacts.  But getting the location right requires experience, and if you have to bring out an electrician anyway, they'll prefer to replace.

Do this with the power off by the way.  And it's far better to take it apart and lubricate, compared to the squirt and hope method.

Consider upgrading to a dimmer model with a remote control, CFL compatibility, or other features.