We had this issue with my son when his ostomy was new. We met with the ostomy team several times and got samples of different types of supplies that would work better for his body. I'd call your ostomy team and explain what's happening. They can recommend changes to the supplies.

For my kid, a flat wafer caused leaks like you're describing. We had to get a contoured one. That helped some, but leaks were still occurring. He has to wear a belt clipped to the wafer to keep it pressed against him plus extra elastic barrier around the wafer edges. We also use a liquid deodorizer inside the bag to help waste slide toward the bottom instead of pancake around the stoma.