I have two AHM diesels in use on my layout-a GP18 (well known from my last post about it-Advice on an AHM Geep 9) and an SD40. I got the SD40 up to 13oz, but I still need it a tad heavier to get good electrical contact, however it works as well as you’d expect. The Geep is the same way, except I haven’t added any weight. I’m wondering how to add weight and make it not so visible.
Lead is your best bet for ballast. You tuck it in anywhere it fits. Silicone bathtub caulk will hold it in place. More is better.
You add weight to increase pulling power. To improve electrical contact, clean the wheels, clean the track, and clean and polish all sliding electrical contacts inside t he locomotive. For instance the trucks usually carry juice to the frame. Clean the bearings upon which the trucks swivel.