Well, I put on 6 kilos in 4 weeks and I was using protein (and I work hard in the gym). Probably not that healthy, but saying they don't work in assisting muscle recovery is a bit of a stretch.
If you want to increase your bench, drop it down to 30 kilos and start doing 5x5 (5 reps for 5 sets), do your bench twice a week, and every time you bench put the weight put by 2.5kg-5kg (depending on if your gym has 1.25kg plates) - so one week you'll jump from 50 to 52.5 or 55. If you can do 5x5 of a weight, just move up weights.
There also might be a particular area of your muscles that's weak, and also depending on how you hold the bar. I mean, if you have weaker shoulders and you push the bar up and down between your throat and nipple area, you'll stop gains pretty quick. If you hold the bar over your nipples or just under them and have a bit of a wider grip, then you'll activate your chest a lot more. Think about where you're gripping the bar and where you're holding it in relation to your body. I'm pretty confident you'll have a particular muscle group letting you down (probs shoulders if you're a newbie)