My answer will be a little redundant in comparison to the thread you reference, but I'd like to put some thoughts together in one post.
Unfortunately it's impossible for us to determine the exact cause of these problems without actually seeing the housings. Things to look for that could help us identify the source of the problem are:
1. Is the shutter control actually hitting the shutter button at depth? After about 15' the shaft is pushed in about as far as it will go. I'm not saying it's impossible, but it is unlikely that there is a mechanical issue causing the control not to hit in the 100' range if it works in the 30-50' range.
If the shutter button is able to be depressed but the camera will not take a picture, proceed with these steps:
2. Confirm that no buttons are being hit by controls on the housing. The pushbuttons on the top of the D90 camera (exposure comp, metering, etc) are common culprits.
2. Use manual focus. Does the problem still occur? If this resolves the problem then it suggests that the camera is not able to autofocus at depth. This is usually solved by adding a brighter focus light. If it won't autofocus at all underwater and you're using a dome port, it would suggest that your lens requires a diopter.
3. Does it take a photo with the strobe(s) turned off, but not with them turned on? If so, then return all electrical components--housing, strobe(s), and sync cord. Again keep in mind that you may need a focus light to try and take a picture without strobes at 100'.
4. Does the top LCD give an error code when it refuses to take a picture? Take note of it.
And in response to Lee, the corrosion of the pushbutton could definitely keep it from returning properly at depth. Please return it for replacement of the corroded control.
Regards,
Jean / Ikelite
IKELITE Underwater Systems