what causes the headlights not to be able to swith form dim back to bright

Most likely a worn/broken switch.

This year group of Volvo has a problem with the ECM (electronics control module). An internal computer malfunction is at fault. It's located directly under the steering column to the left. Just take out the lower panel and you'll see it. Do a web search for what the part looks like. You should only need a phillips head screwdriver and maybe a small pick the unclip the wiring harnesses attached to it.

Make sure that your headlight selector switch is set to the daylight running position (from off, turn to the first position). This will allow you to use your hi-beams as you wish. If you select the headlight position (from off to position two), your hi-beams will turn on, but turn off as soon as you release the stalk.