The problem is that the CBU's use a radar altimeter too, and their fuze function altitude is likely coming up before your desired opening point. You can carefully select your ingress to help with this, or use a timed release mode instead ... the latter is difficult.
You can also attempt a steep (45 deg) angle dive-bombing.
Sure this is in the manual somewhere, but in hilly areas I have a hard time with CBUs opening well short of where the pipper is when I pickle. Am I missing something? The radar altimeter switch isn't messed with or anything, does the jet think I'm CCIP bombing targets that are at a different elevation or something? Thanks.