Did this unit ever work? I gather it did at some stage otherwise it would be a warranty issue. So if it did work at some stage, what has changed to have it stop working? Just looking at the triac circuit is probably not going to help solve your problem - we have no idea of the rest of the connections.

When things have been working for some time and then start to fail, you have to look at possible causes. It seems to be human nature to start looking at the items we least understand (Russell's law#1) where you really should be looking at the things you understand and can test. How about the motor? Maybe the bearings are failing so the motor has a greater startup current? What about the stuff the motor drives? Has the triac been changed for a different type? Have you checked C1 & C2 to see that they are actually still functional - if in doubt, replace them as they are cheap items (be sure to use X2 rated capcitors). Check the motor start and run current. Check the motor start caps (these can degrade over time). Look,feel,test anything you can. If you really, really think the triac circuit is faulty - substitute a solid state relay and see if that fails - if it fails also, that basically proves your problem is most likely elsewhere.

When repairing stuff, it helps to get an idea of the history of the problem. If something worked for 10 years, failed and was repaired but it keeps on failing (very common) suggests that either the original problem was not fixed or that the components that were replaced were not the correct ones. Put it this way, if a fuse blows - is it a problem with the fuse or the circuit it is protecting?