Well mate it's a really common problem, many guys have been in this situation.
Thing is, if what you're doing now is not helping you toward your goals, then something (or some things) need to change.
Building mass is a function of many things, the major factors being:
Sleep - very important for building lean muscle mass. During sleep your body secretes a host of hormones responsible for building muscle. Aim for 7-8 hours each night.
Diet - your nutrition is extremely important and complex. Without going into too much detail, you need good quality carbohydrates, proteins and fats in your diet. All three are equally as important to consume...if you neglect any one of these then you will not build muscle mass, period. Aim to stick to natural foods (meats, eggs, dairy, fruits, vegetables, grains) as much as possible.
If you are maintaining your weight, then this may mean that you do not have enough of a caloric surplus in order to gain weight. So, you need to eat more. Eating more is not just eating a Big Mac....but rather eating more good foods, in their respective proportions, in order to gain mass.
Timing of meals is important too. Aim for multiple meals every day (5 absolute minimum). Fast acting proteins and carbohydrates are very important right after your workout. This is where you may want to consider a protein shake.
Exercise - okay, you can have the sleep and diet in check, but if your exercise component is lacking, then you're not going to build muscle mass. You need to ensure that your training is conductive of your goals. For example, bicep curls every day, 7 days a week is going to be about as effective as not performing them at all with respect to building muscle.
That's everything in a nutshell. I would recommend that you read a series of articles I have available at:
This may be a good starting point for building mass.
Also try searching up "build muscle" on the amino z website...an array of articles and Q&A's available.