[Andrew]

I think ultimately it suffered when the series was dropped and they concentrated on the specials instead - means they somewhat become a tickboxing exercise without the overarching elements a series enables.    It's fair to say as well that they're not the first comedy to suffer in that manner, although of course series becoming just Christmas specials like Only Fools and Horses and The Royle Family tend to have passed their peak as well.    When you look back at the classic Christmas specials over the years it's often the ones that aired whilst the series was still airing which are the stronger efforts.

Yes with the specials they feel the need to include every character so as not to snub anyone for the whole year’s episodes (until of course they started deliberately snubbing some characters after a fall out)

So you’d have a main plot (paper thin in itself) and loads of sub plots, and many of them never actually go anywhere and just fizzle out half way though 

The Royle Family suffered because when it was only specials, they felt the need to do something special, and as a result outlandish and far fetched. When the Christmas episode was just another episode alongside the normal series, they could just have them being the same as normal, with hardly anything happening, which everyone knows is what The Royle Family was all about.