Newbie Need some sound advice

Greg here
I am a male 1.72m weight 73.25 kg muscular body fat is at 18%.
I have been weight training g for the last 4 years 4-5 times a week.
I have a good diet plan and I try to intake about 150 grams of protein a day.
I have a good muscular build but I am not gaining weight or extra muscle
What would be the best course to go on and for how long?

Please help