Is the 900 not featuring Iron Bias adjustments like the 600 and 800 has? Iron Bias is a very picky thing to get right in some areas. Sensitivity too high also causes issues. Of course I'm sure the OP tried adjusting those things. Every detector is different, and the ones he had before he spent many hours with. Chances are he felt just as perplexed in the beginning with those too. But these newer detectors are more powerful, have more options, and are way more sensitive/quirky at times. That's what I've seen. More sensitivity does NOT equal best performance. You have to work within the environment you are in. In very trashy places you have to turn it down until you pick through some of the garbage and REMOVE it from the environment. Once you do that and things run quieter, crank sensitivity back up.
Hang in there and try not to feel overwhelmed. It takes time and that is part of the beautiful journey with any new to you machine. Worst case, you will not "jive" with the machine and it will not be a good fit. It happens!