Canon S95 Manual Help

I'm pretty new to this forum, and I'm hoping y'all can help me out. I have been shooting with my Canon S95 underwater for quite some time now. I've done quite a bit of reading and research amongst this forum and I have learned a ton of information. My problem is when I'm shooting in manual mode with the Canon S95, my strobe (YS-01) only fires every other or every third photo. I can see the on board flash but the strobe doesn't always fire. I have tried setting the on board flash to low, medium, and maximum in Manual mode but with the same results. When shooting in AV mode, the strobe ALWAYS fires. But I can't seem to get the strobe to always fire in manual mode. If it matters, I have the camera in a Recsea housing and fiber optic cable connected to the strobe. Again, I'm pretty new to this so any help at all is much appreciated.

I shoot the S90 on manual with an Inon D2000. Here are some tips based on that.

Yes lower the in camera flash output using menu.

I think the main issue is whether your strobe is shooting a preflash and the camera cannot cooperate with that? Sounds like the camera is skiping over a preflash?

The S90 is a preflash camera in any Auto mode, but in Manual it does not work with preflash. Can you change the Sea&Sea output?

I am assuming that the S95 is similar to S90, but believe they are.

Disable red eye pre-flash in s95 setting. When shooting Av Tv P or auto the s95 flash shoots TTL so set the ys-01 to TTL. When you are in manual set the ys-01 to no pre flash. The symbol is the one with one lighting full am done empty

Wow, that was really an easy fix. I can't believe I didn't think about the pre-flash setting. Thanks for pointing out the obvious for me! I'm sure I'll have more questions in the future, but for now it's time to get underwater! Thanks again