Kind of hard to figure out the issue from your description, but I'll offer a few things that may be contributing to it.
The Valsalva involves forcing air against a closed glottis...it's not just squeezing abs. I don't know if you're doing that or not.
I don't know what you mean by a "big belly breath." If you mean you're only "breathing through your stomach," I'd move to a full breath. You'll notice that some of the lifters on this forum look like they're taking a massive bite out of the air during their breath.
I also don't know what you mean by doing the valsalva "before getting under the bar." If it's a squat, you'll get under the bar, take a breath and tighten everything so you can unrack the bar and take a couple steps back. Then you'll reset your breath, valsalva, and do a rep. Then you'll repeat that four more times. If it's a deadlift, you'll have to do your valsalva-ing before each rep.
Also, do you use a belt?