The weights are not “challenging” enough.
As a beginner, your coach or fitness trainer may have provided low weights for you – like 5-lb. dumbbells and 10-lb. barbells. This is on the right track, if you ask any fitness expert but if you want progress and development, those 5-lb. and 10-lb. weights must be increased, eventually.
The best way to know for sure is to practice the mentality – “if you can, you must”. After 3 weeks or so of lifting the same weight, try to move on to the next level. If you can feel the muscle doing its work with a bit of difficulty at the end of your set, then, that’s your next target weight.
Why do you need to do that? Remember what I said about feeling the muscle doing its work? Well, if there is a bit of strain on the muscles while you lift it, it means that there will be some muscle micro-tears. This is what you need to bulk-up or to rip it (hence, the term) because the muscles break down, repair itself and add density.