If changing the magazine did not solve the problem, then I suggest that you take the rifle apart and give it a thorough cleaning. I have a 7400 also and it only started to perform well after being well cleaned. Just like yours, it would fire the first round but the bolt would then not fully lock up after cycling, leading to some FTF.
For some reason, some of these Remington 7400 type rifles tend to be picky about being cleaned. My brother has a 740, we hunt together, and he never had FTF issues with his...