There are a number of reasons - climate change is one, desert is advancing in some areas at an alarming rate - we dont see the effects of climate change so much in the UK but the Saraha for example is expanding south at something like 6 miles a year.
There are also population pressures - there are a lot more people competing for the water.
In some areas there are development pressures etc.
Some countries like Saudi used to rely on "fossil" water laid down thousands of years ago in the ground which is running out.
The issue about well building I guess is expertise - if your village had a well for 100 years and it dries up do you know how to build a well?
It's not just a case of digging a hole - where do you dig it? is the water safe? how do you get the water out? how do you stop it collapsing in on itself.
try here:
http://www.wateraid.o...hat_we_do/default.asp