This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
This question regularly arises on the newsgroup.
Real ales, such as those mentioned above, are living products, produced
using natural processes (fermentation) from natural ingredients.
Despite the best quality control systems, some slight variations in
flavour, and occasionally colour, do occur from batch to batch, from
season to season and from year to year. Factors such as the quality of a
particular year's hop or barley harvest, the length of time since the hops
were harvested or the barley was malted, even the weather can affect the
end flavour of a beer brewed to the same recipe.
This slight but perceptible seasonal variation in a naturally conditioned
ale may be thought of as similar to that found in the finest vintage wines
and is considered by some connoisseurs to be part of its attraction.
Generally, cask-conditioned beer from the larger-scale and more
'industrial' breweries is more consistent than that from the smaller craft
breweries, but the down-side is that it is often less interesting.
Furthermore, ales with complex flavours are possibly more prone to
variation than those with a relatively simple taste profile.
The issue is compounded by the way real ale is now distributed around the
country via wholesalers and sold far from its home territory using
dispense methods other than that for which the beer has been brewed.
In addition, different beers often need to be handled differently in the
cellar. Some benefit from being allowed time to mature, while others may
not need this. A licensee having a particular beer as an occasional guest
may not know how best to look after that beer.
As a result of all these factors, a particular beer may taste different
and not as good as the drinker fondly remembers it, particularly if they
only manage to sample it occasionally.
Some breweries do change the recipes, lower the gravity or otherwise muck
about with their famous (and not so famous) brands and where this is
happening, or suspected by regular drinkers of the beers, the newsgroup
welcomes postings alerting us to the problem.