From Tony Scott in today's NYT:
The word "unfunny" surely applies to Mr. Myers's obnoxious attempts to find
mirth in physical and cultural differences but does not quite capture the
strenuous unpleasantness of his performance. No, The Love Guru is downright
antifunny, an experience that makes you wonder if you will ever laugh
again.
The funny's a tenuous thing; you can lose it at any point and similar to one's virginity, once it's lost, it takes a helluva magic trick to get it back. It's as close to being an athlete as any other human endeavor in that quality; except athletes have predictable career arcs and a decline that usually occurs over a protacted period of time. The funny can leave anyone at anytime. I don't know that it happens to everyone; George Carlin, let me suggest, still has the funny, which is why, based on career value, were one doing a ranking of stand up comedians in the way that I'm currently ranking baseball players - you'd have to rank Carlin at #1 ahead of Pryor; you'd like Pryor for peak value, but Carlin runs over him in terms of longevity.
There's a more systematic way to approach this; really, what I should do is do some research and chart this more methodically, but I feel comfortable in saying that you've observed the same phenomenon ; The Case of the Disappearing Funny. Robin Williams, for example, once had funny and no longer does. Same with Billy Crystal. Makes you wonder about the funny sucking powers of Comic Relief; I wonder if you went back and looked at the tapes, could you see the precise moment when the funny left each of their bodies; in my mind, if you watched it frame by frame, it would look like when Buffy would stake the vamps - you could see their funny just turn to dust.
I'd throw Whoopi into that basket too, but she's never had the funny, let's be honest.
If you can't find all of those old Comic Reliefs on the youtubes or the whatnot, consider Bowfinger, a set on which both Steve Martin and Eddie Murphy clearly left their funny.