You didn’t mention what cranks you’re trying to use. Not everything is compatible with every crank.
Most recent SRAM MTB cranks can’t be used with crank arm power meter because they don’t have any flat surface on the backside of the crank arm. So I had to do a crank spider solution. I ended up using Sigeyi and I’m satisfied with it. The only real downside is them charging extra to unlock the more advanced software features, but they are up front about that. [Edit: not true. That’s the Power2Max NGeco].
If you’re interested in it watch GPLama’s video