If you are gonna do the step up method, and do another shot if you don't feel the first small one, please make sure to re-register a couple times while injecting. If you're gonna do another one within a little while after the first, you've gotta make SURE you hit.
By re-registering I mean, after you first register in vein, by seeing that shot of blood, inject 20 or so ml's, then pull back and see that blood again. I usually do it twice per shot. That way you ensure you haven't either poked through the vein, pulled out, or had it roll. I've seen shakey hands lose it many times.
Also, if you feel burning at any time during injecting, stop, and make sure everything's right. You should not feel a burning sensation.
I say this because dropping a hit shot on top of a missed one could be real bad. That in vein shot can end up rushing through your bloodstream at the same time the missed shot is starting to seep in and hit you as well. If the shots are weak enough, it might be OK. If not..... Be careful with that shit. We still don't know what you're dealing with.