It sounds like you're using the retractable needles, which are a pain in the arse. You have only one chance to push before it locks in place and you have to change up the needle. There's a few times I wasn't certain the patient got the entire dose, and like you said it seems like some fluid remains on the surface of the skin. I found that pinching the skin fold to stabilize the area helps and, as already mentioned, hold the pen in place for a few seconds after administration. It requires a much firmer approach than the regular needles.