Not all 9 volt batteries are the same size. Though there is an standard size for 9volts which is H 48 mm × L 25 mm × W 15 mm. However these standards still allow for size variance for a 9 volt battery. The maximum size permitted, for example, is 48.5 mm × 26.5 mm × 17.5 mm. So not all 9 volts are made with the same specifications - cheaper batteries will be bigger because they use cheaper materials. Thus some 9v will fit into some loader/grips and some will be a tight fit.
A good tip for 9v batteries is to keep using the same battery brand for the best results and NOT to change brands. Switching brands (and sizes) could damage your loaders and circuit boards in your markers. If you can get an Energizer/Energy 9v to fit into your grip- then stick to it.
If your 2 9 volt batts are getting jiggy in your Vlocity - My tip is to trim and fold foam sheets a few times and stuff it on the side. Same goes for the 9v in your grip. Or a small piece of sponge would also do the trick. I like using sheet foam because you can roll, fold and trim to get the desired size. I wrap my 9v batt in my Pulse with a strip of foam sheet that gives just enough size to stop it from jiggling around in the bat cavity. You find foam sheets used to wrap fruit or electronic parts.