You are not tracking your progress
One of the most basic principles of gaining muscle is to always progressively overload. This means that on a weekly basis you should have progress, that means increasing weight, reps or sets and another way is to reduce rest time on each set. Your muscles need to constantly adapt to new loads and stimuli in order to increase their size, strength or endurance. So for your next session bring a notebook and write down each set, rep and weight you lifted in order to lift more the next time you go to the gym.
You are prioritising strength over hypertrophy.
You are obsessed in lifting your first 225lbs(100kg) bench press. Therefore, you are neglecting your Hypertrophy sets (6-15 reps) and focusing solely on your strength (2-5 reps). This won’t help your chest grow. Studies have shown that muscle strength does not necessarily translate into muscle size. In order to actually grow your muscle fibres you need to focus on muscle hypertrophy while also increasing your strength through low repetitions and maximum weight. So next time try adding in some high rep sets after doing your regular strength sets.
You are in a deficit
Being in a calorific deficit means that you are eating less calories than what you burn in a day. Usually when you first start training, you see huge results and fast even if you are in a deficit (body recomposition). Unfortunately this is short- lived and only lasts a few months and in some cases for a whole year. Once you hit your first plateau, that is when you should start taking your diet a bit more seriously. Your muscles won’t grow any further without the help of calories. Try increasing your calorie intake by 10% of your maintenance amount. For example, if you burn 2000 kcal a day, start eating 2200kcal so you start making muscle gains again.
You are overtraining
Their are many people in the fitness industry that unfortunately do not believe in overtraining. Studies have shown that their is a certain point in terms of volume where your body undergoes mechanical damage and goes beyond training to gain muscle. That point is somewhere between 20-30 sets per muscle group per week. Depending on the person, genetics and years of training experience this can vary, but anything over 30 sets might be a bit too much. Your classic bodybuilding routine (Chest, Back, Arms, Shoulders,Legs) in enormous volumes might work for pharmaceutically enhanced individuals, but for everyday folk like us this isn’t the smartest way to train. Try splitting your routine into
Upper/Lower/Rest/Upper/Lower/Rest/Rest or Push/Pull/Legs/Push/Pull/Legs/Rest while taking into account the amount of volume doesn’t exceed 30 sets per muscle group per week.
Genetic factor
This is a factor that unfortunately cannot be changed. If you are lacking genetically it will be hard to get that plate of armour of a chest. But that should never stop you from trying.