From what I read it should be on the first character you loaded up. So you will only get one and it'll be the level of the character you first loaded right after you put in the code.
Mine was in my backpack so I don't know what the problem is unless you're thinking all your characters gets the shotgun.
It's a side mission reward which can still get it. It's the mission where you have to retrieve Makus's safe and you will have to turn it in to moxxi. The valentines special is the critical version of the shotgun.
There have been others with the same issue.
Here is the thread for the Gearbox forums. I didn't read through all of it, so there may be an answer in there, and if you stay updated on it, an answer may show up. Other than that I can't really offer much help.