When you miss an IV shot of heroin, it can depend on how much of the dose went into the vein and how much didn't. Usually, even with a missed shot, you still get a little bit IV and feel that almost right away, and the rest of the dose will come on within about 5-10 minutes.
You can easily tell if you injected right under the skin when a bump begins forming very clearly on top of the spot where you're shooting into. You'll also be feeling some pain and pressure at that spot. If you are injecting into a muscle, there won't be as obvious of an obtrusion, and depending on if your shot is prepared correctly and if you're going into suitable muscle tissue there wouldn't be an immediate sign. If it went well it should absorb and start entering the bloodstream slowly.
All of a heroin shot will eventually make it's way into the bloodstream, it just depends on whether it directly enters it or not if you feel the effects very quickly or slowly.