While there are many reasons for comedy series to become stale, but a primary one is that when they rely on a single idea or joke for too long with no variation, the series in question becomes repetitive and boring after a while. For me that is exactly what has happened to Cinema Sins.
Before I complain, let me just say that at its inception, this was a great series. The unique idea served itself well in its early days, with interesting critiques of films with the crew finding no stone unturned when it came what is wrong with major films, both the huge (plot holes, bad writing) and the minimal (extras messing up, miscalculations involving time or science). It also helped that the series was quite funny. Now while the writers aren't the best comedians in the world, nor were they as funny as similar series around this time (Honest Trailers, How It Should Have Ended). However, the humour they managed to get from the mistakes in these movies could be at points extremely funny, mainly the ones where they delivered it via sarcasm. In short, it was a very enjoyable series initially.
But I will admit I don't watch the series much anymore. Why you may ask? This is because for me, the show has remained largely the same throughout with very little change or variation. Now I understand this can benefit sometimes, since if an idea ain't broke, don't fix it right? But for me, while the concept behind Cinemasins is still an original one, its lack of range is more of a hindrance than anything because it becomes rather boring and repetitive after a while. The occasional spanner in the works while very entertaining and enjoyable (mainly the ones with guests and the one where they mock themselves) aren't frequent enough to break this cycle up.
It doesn't help that the humour of the series is inconsistent at times, and on the whole very safe with no edge. Now, I have to say that I have found some older episodes quite funny and I don't expect every comedy series to be Frankie Boyle levels of edgy. However, the humour derived from the nitpicks can be quite poor at points, not helped by how as I said earlier, the writers aren't the best at comedy. This lack of edge doesn't help because it means the humour is very safe as a whole, something which would be fine if it funnier than it was. This can lead to episodes that while entertaining and offer insightful criticism of certain films can be very hit or miss in the laughs department & a bit of a drag as a result (their recent Alien episode is a good example of this).
So as a whole, Cinemasins for me is one gimmick stretched for longer than necessary. It's repetitive formula can get boring after a while and there are laughs, they're not as often as one would expect. So they're entertaining critiques of big films, but whose premise has had so little variation over the course of nearly 5 years of existence that there's not much to reward regular watching. Nostalgia Critic creator Doug Walker once said that one of the reasons that he initially stopped the NC was because he had felt he had run out of ways to say a film was bad. Unfortunately, while Doug's show was at least versatile enough to keep the viewer engaged and wanting more, Cinemasins has had so little change that it has led the viewer to become disengaged and turn off. The occasional episode is still entertaining and can be quite funny, but the show has now become like fast food. Fine in infrequent doses, but not rewarding in frequent ones.