On off switch - Have had shower for about 5 years. No - soft water area. Shower works intermittently but sometimes have to press the button a number of times to get it to stay on. Temperature is always set at warm.
Have you had the front off to see if the switch that the button operates feels iffy, or if there's any visible problem with a linkage?
You could try the shower doctor website, but it might just be that at 5 years something needs replacing. Whether the price of parts, and the cost of paying someone to fix it if you can't DIY, are worth it only you can decide.