Could be one of a number of things.
First thing, though; what kind of video mode is the OSSC outputting, and does your monitor actually support it? From what little I understand of the Amstrad, it outputs at or about 50Hz, which the OSSC won’t modify; and what documentation I found for the BenQ BL912 (the resolution document, specifically) suggests that it doesn’t support 50Hz.
If your monitor does support 50Hz, then I would think that the output of the Amstrad is too off-spec for the monitor. What does the LCD on the OSSC report when your Amstrad is powered on?