@blueskies_up_ahead do you recall where on the rig the weight pockets were located? People often place them on the tank straps, or the waistband webbing. If they were on the tank straps, try moving them to the waist band. If they were already on the waist band, try using a belt instead, and put the lead as far forward as is comfortable. If all that fails, then either an aluminum plate or a less buoyant tank would allow you to move more lead toward your belly, like MichaelMc says.
When you adjust your shoulder straps, ideally you want them to be loose (loose enough that you could slip your arm out without loosening anything, if you wanted to). But the waistband and crotch strap should be tight -- not uncomfortably tight, but with very little wiggle room. If your crotch strap was too loose (or if you weren't using one), then when you roll your body, the tank will have a tendency to slide sideways along your back. If that is happening, dialing in the crotch strap length will help.
The other thing that can help is body position. Ideally your hip joint will be straight, and your knee and ankle joints will be close to a 90 angle most of the time. This brings your buoyant lower leg, ankles, and booties up to the same plane as the negative tank, which will help stabilize things. If you already have that body position while in normal trim, but have a tendency to straighten your legs out when you roll your body to the side, then it would contribute to roll instability. So try to keep roughly that leg position even when you're sideways, assuming you have the space to do so.
Hope this helps, cheers.