Welcome to the forum!
I have this problem sometimes too. My husband can do the tent, but I can't seem to grab enough that way and I feel like I'm shooting more blindly since the syringe ends up buried in the fur tent. So I tend to just grab whatever I can and roll my wrist or lift it enough that I can't shoot into his muscle. At our last vet visit I checked with him about it and he said it was totally fine. And it gives me a much better visual that way so I know I haven't hit a blood vessel.
I find if you can get kitty to lay down it can really help. I can't grab enough when my guy is sitting up because his skin tightens. I'm super lucky that my guy now jumps up on the bed for his shot and lays down. He didn't at first, but after a few days he figured out the routine and now he lays down and waits for his shot and treat!
The other thing I've noticed is that depending how kitty is laying, one side is often easier to grab than the other. If he lays straight on (meatloaf position, like my profile photo) then either side is the same, but if he's leaning a little more on one side, the bottom side can be a little easier to grab as the skin has a bit more slack. We try to alternate sides with each shot, but sometimes we just have to adjust based on how he decides he's going to lay down for it.
Wishing you and your kitty all the best!