My '90 740's windshield wiper nozzles are forever clogged leaving me with a perpetually dirty windshield and lots of aggravation.

When I turn them on, what comes out is a little dribble on the hood on the car, never quite powerful enough to even reach the glass. I've tried using a can of compressed air to blow out anything from inside it, as well had a mechanic use their industrial grade air gauges to 'clean' it out. Once he did, it worked fine for about a week or so. Now, it's gone again.

Any other approaches to solving this problem?
I've check the hoses leading to the nozzles, they appear to be leak free, and the water seems to be flowing with good pressure (since they did work when the mechanic's compressed air was used).