I think sometimes (and I admit, until I realised it was a key failing, I was often guilty of this) the GM comes up with a really interesting or original concept.  It is incredibly detailed and seems well planned out and attracts a decent group of RPers who make it through the acceptance stage without dropping out.

...And then the GM realises they haven't come up with an ending or a path to the ending or a set of events to follow.  This results in a bit of playing by ear which sometimes results in some fantastically original and imaginative scenes and plots but largely ends in a mish mash of undirected posts clashing against each other as no one knows quite what to do.

Also, a lot of people RP seasonally.  It's winter and the days are short so there's less to do so they stay at a computer at home and RP.  Spring comes: hurrah!  We can now go outside - and activity dwindles as other commitments come to the fore.  When this happens I often take a small leave of absence (once I'm sure any RP I'm a part of is simply not going ahead and the GM has called quits) from all RPing and return with a fresh outlook.  After fifteen years I think this has worked best for me and stops me from being jaded... well, more jaded.  I'm British, we're born jaded.