something i can help on for a change with some personal experience that may help you.
1) i would have the metal detector health checked by the manufacturer, i recently had Loma out and it cost next to nothing for a 1 hr service. which they generated a full report on whether or not the detector was working / worn out due to age ( it is a 8yr old machine )
2) they also could informed me that the installation settings logged for the machine were not right, which are not always the same in all environments, temperature and humidity for example and types of product to be scanned can be setup differently.
3) has it been moved about? has it had anything attached to it since installation? an engineer fitted the control panel too close to the aperture of the head causing false readings when it wobbled about or somebody walked passed the machine.
4) is it the right machine for the product you are detecting? smaller businesses and i am not saying yours is tend to not want to use multiple detectors for different product lines due to cost and will end up with a big metal detector being used for tiny products and vice versa, (this was happening where i am and rectified it, the detector worked fine but but again hard to setup and keep stable sometimes.)
5) presuming your packaging is not foil based? if it is you shouldn't be trying to detect stainless anyways. ( again i worked for a company that was trying to detect stainless and when they couldn't get there brand new detector to work MAGNETIZED the ball in the test piece to make it work, i don't need to say anything more as to why this was a bad thing)
6) is the belt clean? has it been removed and cleaned and subsequently the machine calibrated out to ignore signals from belt?
7) any other electrical equipment near by that could interfere with the detector? remember the detection is not just under the metal aperture its around it (magnetic fields) i have known some detectors to be so sensitive they react to a mobile phone in pocket or even a watch.
8) my 2 personal favorites. a) paperwork / clipboards near the machine, padlocks on lock boxes that are magnetic in nature can affect the machine if close to the head and b) is the right program being used? and are those settings correct for the product? some company would have a hard copy program folder with all settings for a specific product so they can be checked when something changes like a raw material is not quiet the same as it was last time.
9) are the test pieces the right size for the product type?
10) cant think of anything else of the top of my head but don't like to end on 9..