Sometimes the probe itself can get plugged. If too much or too heavy lubricant has been applied, ports may gum up.
Multiprobes are easily damaged. Borrow someone else's probe and fly with it.
Or test with your own then with the borrowed one.

While fixing problems is more difficult than single probe, testing a multiprobe isn't difficult.
Essentially use a syringe and the ASI alone (as in Reichmann's book) to test each of the isolated lines for leaks, with probe attached and ports sealed.
Check for crosstalk between circuits. Pitot is the thin port at the end, static at the change in tube diameter, TE the hole in the side.

One day in the shop we found two bad multiprobes out of five tested. The other three probes worked on all three gliders. Glad it wasn't the mounting sockets!
It doesn't take much leakage to throw compensation off.

Electronic TE works very well. Personally used it with Cambridge, LX and ClearNav varios. Some varios require TE connection be connected to static. Some (ie: ClearNav) don't, so you can try both settings in the air.
Many of us have used electronic compensation in gliders that don't have a multi-probe. The 302 in my LS6a worked remarkably well with electronic compensation.
Jim