Insufficient food could certainly be a part of this. I've seen younger lifters really benefit from using whole milk and when I was younger I'd do some fruit, a couple of cups of whole milk and protein powder a couple of times a day. This helped me put on about 20# over the course of a school year and my lifts went up significantly. The thing with adding certain foods in, is that not everyone reacts the same to them so go slow. If you're not drinking a lot of whole milk don't just start pounding a gallon a day of it. Start slow and see how your body reacts. If you notice adverse affects (bloating, gas, congestion, etc.) and your body does not adapt to it, milk may not be the answer. But honestly, for some it works like magic and is pretty damn cheap.
The other thing I've seen with teens when their progress stalls is related to growth spurts. Guy comes in and lifts are strong, form is great then a few weeks later he's had a growth spurt and everything has gone to shit. Why? His body changed enough and enough mobility has been lost that he simply can't move right. So, we drop the weight down, fix the form, loosen up the soft-tissue that couldn't adapt as the rest of the body grew and work the weight back up and beyond where they were. In these cases, it's not a matter of strength or a plateau in the normal respect, it's simply a change in the body happened and now they can't move the weight properly. Usually this is most obvious in the deadlift and squat, but can happen in the bench and press as well.
If you post what you're eating for a couple of days (including rough quantities) or videos people here can probably help you more.