This is a problem I observed and helped solve on a liveaboard a month ago. Another diver had a DSC-F717 in an Ikelite housing and using an Ikelite DS-50 strobe. This system uses a sensor inside the housing to detect the camera's flash.
He was complaining that his external strobe wouldn't work all the time. I initially asked him all the standard questions about having it in forced flash mode, having the strobe turned on, etc. It was clear that he fully understood how to operate his camera/housing/strobe. He said the strobe worked on the surface, it worked above 20 feet or so, but wouldn't work when deeper. He later proved it to me by showing me on the boat that it worked then showing me again at depth that it didn't. He and I went over and over this problem between dives, concluding of course that it was pressure related (effectively the only thing that changes from 20 to 70 feet).
Eventually after looking over every cable/connection/setting and putting the camera in and out of the housing a couple dozen times, we noticed that the pop-up flash touched the top of the housing. We hypothesized that the housing compressed when at depth just enough to push the pop-up flash down, turning it off. The owner discovered that the plastic "shroud" on the pop-up flash would come off by removing two tiny screws. This would lower the flash's profile by a few millimeters. He took it off, put it all back together, and presto, 100% strobe function.
Hope this helps save some shots for someone in the future.
