Do you have a sensitivity to Vitamin B3 (niacin)? Fiasp is just Novolog mixed with that. Since you are OK on straight Novolog, beyond from the obvious comment, “Stick with it”, it might be worth checking out the possibility of some reaction to the extra ingredient.
For me the extra maybe makes the expected difference, but all we are talking about is a 5 minute, 10 minute absolute max, acceleration to the start of adsorption of the insulin. This might make a difference to me because I only know how much I am eating when I start eating. However in many cases I need a 20 minute warning to manage the post-prandial highs. In some sense it is a waste of effort; an extra 10 minutes really isn’t enough.
I use it because it doesn’t cost me any more and, apart from the odd stinging which isn’t an issue for me, it has no downside. I don’t care about the stinging. If Novolog cost me less (impossible for me) I’d use it instead.