Tritium sights have a usable life of around 12 to 13 years from the time of manufacture. There isn't anything you can do, to "rejuvenate" them, short of replacing them.
You might contact Sig CS, and advise them of this situation. You didn't state the date of manufacture, if it's been sitting in a warehouse, or shelf/display cabinet for years, they probably won't do anything. If it is of recent manufacture, they may replace them, but you won't know until you contact them.
Sig advertises, installation of SIGLITE night sights on most models for $119.95.
There could be more visible sights available, but most any tritium sights require pusher type tools to install and remove the sights to prevent damaging the gas filled capsules.
Trijicon has a High Definition set with either Orange or Yellow enhanced front tritium capsule. Tru Glo has a enhanced front including Fiber Optic surrounding the tritium capsule, in their TFX Pro series.
If I were you, I would check with Sig to see if you have any recourse on their "dime", if not, then I would decide what you really want, before paying for it yourself.