I see a lot of people on the Internet telling others that they absolutely must train to failure in order to build muscle. This is false. It is absolutely possible to build muscle without pushing yourself to muscular failure.
Sometimes, constantly training to muscular failure can put the central nervous system into shock and stall muscle growth. Likewise, the central nervous system of some people will respond better if not trained to failure, just close.
You see, muscular failure is the absolute end of the line. It is not your muscles which have failed, but your central nervous system.
Proof of the fact that you do not have to train to failure to achieve good muscle mass is the results which can be obtained with compound lifts. Compound lifts use several muscle groups in the body, yet they still make muscles not completely responsible for the lift grow. Yes — even muscles which were not even close to muscular failure.
Occasionally, it by pushing yourself that little bit extra and training to muscle failure you can also use the central nervous system to your favor by giving it an extra jolt to encourage muscle growth.
In future then, when you find your muscle gains stalling even though you train to failure like you always thought you had to, try taking it a little easier for a while. You may just find your central nervous system recovers and sparks off new muscle growth.