I’ve had good results with the Starkey mold lab. The biggest issue is that the hearing professional takes excellent impressions, with top quality material. This should then be overnighted to the factory to minimize the chance that the package is left somewhere too hot or two cold and the impressions become distorted.
Once you get the molds you should have at least 30-90 days to try them and return them if they do not work to your expectations.
The material that the mold is made of will also play a part. Starkey offer a material called StarFlex, which is slightly flexible and becomes a little more pliable once in contact with the warmth of your ear. The softer the mold the better seal you are going to get.
A hearing professional should take into account your preferences, your ear configuration, the type of aid, the type of tube, and the type of loss you have when helping you select the right mold.