Did ReSound make your molds? (you use English spelling so perhaps you are from the UK). I had silicone molds made by ReSound. They stay in place VERY well. I even have to tug a bit gently up and out on the back part of my upper ear lobe to help open the ear canals when I go to remove them.
Here’s a picture and a discussion of mine for someone who was having a problem with Widex ear molds quite a while ago. All molds are not the same. From your impression, someone decides how to make one and they are supposed to consider in planning the mold to be made from your imprecision how they can use your ear anatomy in the shape of the mold to help it stay in place. Some people are apparently more skilled about that than others and as MDB indicated, sometimes it takes a little bit of extra anchor added to the mold to help out. (I was lazy about thoroughly cleaning my mold before I took the picture!). You can also see the Select-A-Vent plug protruding from the lower left of the mold.