I too started having the problem and I experimented with several things to get it where it should be.

I have a2 piece. One being the wafer. With the wafer comes a paper where you match up best size to fit over the stoma.

That way when you put it down on stoma you will cut the right size.
Sizing helps not to overload.

Next before putting wafer down .. if your stoma is clean dust the area with stoma powder and take the bag that is attaching to the wafer and around the rim of the bag lightly run stoma paste. Put the wafer and bag together and paste should cement and hold together.

If you have like I do a fold in my skin you will ; you will place securely over stoma the wafer and bag in place and attach it to your skin.

For added protection I use a barrier extender on my right side of my
Bag/wafer. It will keep stuff from pushing through the bag.

Look up utube. They show you how to do things step by step.

I do hope this helps you.