This is normal weathering. How long has your decking been down? Have a look at any good paint supplier(include the sheds) find a colour that matches what you had, Give your decking a good clean and re-paint. You may need to give two coats depending on degree of fade.
But read through some of the post regarding decking treatments on here. The Cuprinol stain type treatments really do not work very well so you may be better getting a tinted oil type treatment (decking oil) unless you fancy redoing it every year!
It doesn't chip/flake off as it actually soaks into the timber grain and hence is better for extra weather proofing - it doesn't just coat it like some of the stains.
Decking timber is generally pressure treated and is designed not to require further treatment, however, oil seems to keep it nourished and stop it from drying out as much.
Cuprinol sell decking oil in a few different tints at around £25 for a 5 litre can.
If your original treatment was a surface stain or similar you may have trouble getting the timber to take up oil though. This is a problem that a lot of people seem to post about on these forums, as once its been applied, its often almost impossible to use something like oil afterwards, without completely removing it all first.