I've been watching the Big Bang Theory since the very first episode and I realized that the show just wasn't funny anymore. After a few years most shows start to get flat and the characters get tired, but I don't think that's the problem with BBT. The characters on the show are well written and have some depth to them, but I think the problem with the show is something else entirely.
Up until the early 2000's all half hour TV shows ran about 22-25 minutes when you took out the commercial breaks, BBT runs about 18. That's a pretty big time difference, almost half of the shows time is now commercials. I know you're thinking I have a DVR and skip the commercials anyhow, so why does that matter? I'll tell you.
When the show first started there were 5 characters, Penny, Sheldon, Howard, Raj and Leonard. The show usually focused on one plot line and with only 5 characters there was enough time to really get some depth to the episode and make you care. Now they've added Bernadette, Amy, Stuart and one of Raj's on again off again girlfriends. While these are all great characters, there isn't enough time to focus on everyone anymore. You only have an 18 minute show and sometimes there are 3 plot lines going on in a single episode. That leaves 6 minutes per plot line and it just isn't enough time to get a good situation and make it worth watching.
In my opinion this is why the show has lost it's mojo and isn't really worth watching anymore. It isn't the writing, the acting or the characters, it's that there are too many characters and not enough time.
