I'm not that familiar with disc brakes but here goes with some basics:

1. Are they cable or hydraulic?

2. If hydraulic, maybe a flush through or bleed is necessary after a long stand up?

3. Have the brake pads or wheel rims become contaminated with oil or grease? Car brake cleaner might help on this one. 

If they are cable and lever travel is fine then I would remove the pads and give them a good going over with sand paper.

I would give the disc a light rough over with the sandpaper too, just very lightly.

Then find a big hill and get some heat into the brake. If no good then its time to try some new pads.

If you are running flat bars an upgrade to even a cheap hydraulic brake will be far superior in my opinion

It does sound like it could be pad contamination. If you have to replace the pads, clean the rotor and caliper with isopropyl alcohol or brake cleaner at the same time as you don't want to transfer any contamination to the new pads.

If the pads had got severely overheated on a descent, this could cause them to "glaze" which cam reduce brake power and make strange noises. This is easiest to fix with new pads.

Another thing to check is that the cable isn't contaminated with dirt or got rusty. This won't affect the distance that the lever travels but you will probably feel the friction. The force required to squeeze it may be bigger than the force it returns to your hand when releasing.

Thanks David I think I'll sort new pads anyway and clean the rotor.

I was interested to hear on a previous post that hydraulic discs are more powerful,so that might be a long term goal.

Are hydraulic brakes more powerful or just easier to use, that is require less effort on the brake levers?

What recommendations for cleaning discs please, do you have to keep it off the pads or can you clean them too?

Chris, I think a good hydraulic disc brake is more powerful than a good cable disc brake for the same rotor size. If there is less effort needed at the brake lever for the same deceleration, that means more of the force you apply goes into braking (and there is less friction) which means that with maximum effort you can brake harder. However it isn't quite that simple, and additional factors contribute to how good the brakes are. Firstly, the ability to control the power is every bit as important and the "feel" of the brake affects this. Cable disc brakes can be ok, but good hydraulics are better. I particularly like Shimano hydraulic disc brakes from this point of view. Another important factor is the ability to dissipate heat. This isn't about how hard you can brake but how long you can keep braking hard. There isn't necessarily a difference between mechanical and hydraulic in this area. There is the possibility of boiling hydraulic fluid, but your brakes should not be getting anywhere near that hot whether cable or hydraulic. You're more likely to get fade from overheating pads with any recent brake I suspect.

Regarding cleaning recommendations, I use IPA for cleaning discs. There are motorbike disc brake cleaner sprays that should be fine too. It should be fine on the pads - see here (although don't follow the advice on bedding in brakes that is linked at the end of that video, it is the opposite of all manufacturers and Park Tool's advice!)

I would completely agree with David over the benefit of hydraulic brakes. 

As far as cleaning, disc brake cleaner spray on the disc will clean it. 

Although the cleaner won't hurt the pads i tend to avoid spraying anything on them. If a pad is contaminated and sanding the surface doesn't fix it then new pads are required. Once a contaminant has been absorbed its game over.