3 things.
1st, the problem is most likely what you already said, and that is you're not eating enough. Pretty simple, eat more...the best way to do this IMO is to eat more meals, not increasing the meals you already eat. if you are a hardgainer, or you've reached a point to where you are one now, then you may very well be one of these guys who needs to eat 8+ times per day.
2nd, add more fats to your diet and I guarantee you'll grow. Too many people negate the advantage of fats in their diets IMO. many people when they hit a plateau will simply eat more carbs, and to an extent this can be beneficial, but more times then not more fats is the key...I learned this the hard way.
3rd, stay away from slin. You said you used it before and put on fat. Well, this is what slin will do to most people, so they should not be using it. Unless you're a top level competitor making a living from BB, I simply don't see any advantage in using it.