The reason you're not gaining like your 1st cycle is because this is not your 1st cycle. You will never make those kind of gains again...and on your 3rd cycle you will gain less than you did on your 2nd...and then less on your 4th...then 5th..etc....until eventually, you will be lucky to gain a couple new pounds of muscle with each cycle. After that, you will begin losing every pound of muscle you gain from every cycle as soon as you go off. The ONLY way to overcome that and continue gaining more muscle is to take less time off and spend more time on. At some point, even that won't be sufficient anymore...and you will be forced to stay on all year round....and if you want to continue gaining muscle beyond that, you will be forced to use drugs all year long in fairly large amounts just to gain a few pounds of muscle each year.
By cycling 50% of the time on/50% of the time off...you will never approach the size of a pro BB'r. In fact, for most guys who follow that format, their gains will stall after just a couple of years, to the point where they will only be gaining a couple new pounds of muscle each year...and soon after that their gains will stop. Just to give you an idea, by the time you get to around your 6th or 7th cycle (roughly), you will only be gaining about 4-5 oounds of new muscle per cycle...and it will continue to quickly decrease from there as time goes by.
For guys who continue cycling 50% ON/50% OFF...they will always gain a decent amount of weight each time they go back on (a good portion of it will be intramuscular water retention), but as I said earlier, once you reach a certain point in your development, "all" of that weight will be lost when you go off. Your natural hormonal environment can only support so much muscle tissue, regardless of ho good your PCT is...so once you reach the point where your natural T production can no longer support the amount of muscle tissue you built on cycle, you will continue to lose "all" the muscle you gained beyond that point every time you stop using AAS. You will never be able to maintain ANY amount of muscle beyond what your natural hormonal environment can support...period. Unfortunately, the natural hormonal environment of the average man cannot spuport very much muscle tissue at all...not anywhere close to a pro BB'r. Some men are more blessed than others in this regard, but no one will ever get "huge" cycling 50% On/50% off. The BB'r simply spends too much time with an inferior hormonal environment.
As many guys here have already stated, one of the key pitfalls of most BB'rs trying to add size is that they do not eat enough. If a BB'r wants to maximize the amount of muscle he can gain...and does not want to stagnate prematurely...he MUST come to realize that for every new pound of muscle he gains, he MUST increase his caloric intake "forever", if he wants to maintain that new muscle. He will never again be able to eat what he did when he was lighter...and the bigger he gets, the more he must eat simply to maintain that new muscle, let alone gain more. In reality, most BB'rs stop making gains not because they aren't using enough AAS, but because they get to a point where they stop increasing their calories. They think to themselves..."I am already eating so much food...I can't eat anymore"...and then their gains stop. It might not sound fair or fun, but the body doesn't care how full you feel, it ONLY cares if it has enough calories to keep building more muscle tissue. The average 250-300 lb off-season BB'r will require between 4,000-6,000 calories per day to maintain ther muscle tissue. 4K is hard enough for some guys, while other guys have a much easier time with it, BUT...almost everyone thinks it is a total bitch to eat 6K cals per day...and many guys just can do it. Hell, many guys cant even eat 4K cals per day on a regular basis. I have spoken with so many BB'rs over the years who think they eat "sooo much food", but they don't eat shit! When they actually add ther calories up, they find they're eating around 3-3.5 K cals per day. Diet is by far the most difficult part of maximizing one's BB'ing potential. Without a enough of the right food at the right times, the BB'rs is sure to fail in his quest of maximizing his physical potential.