We have a Culligan Reverse Osmosis system under our kitchen sink (not sure of model but can find out). The system came with the house when we bought it. It has never provided a large amount of water but usually enough to fill a glass. Lately, we barely get 6 oz out of it before the flow is reduced to a trickle. What is wrong with it? Does the tank not have enough pressure? Should I just pay for a Culligan service visit?

The tank is probably under pressured. Typically if you turn off the water and open the spout the tank should only weigh a few pounds once it has dispensed all its stored water. If it has insufficient air it will be like a bowling ball (8lbs gal). With spout open and water off you should have the marked pressure on the valve, typically ~8psi. If the bladder has failed you may get water out the schrader valve or with proper pressure it is still heavy. Fill with cheap bike pump.
You should change all the filters and also the membrane if you do not know the last time it was replaced. Filter and membrane problems cause slow recharge and tank issues cause insufficient quantity. Lots of online resources, I have a generic watts premier and have bought all the replacement parts online.
RO systems are usually fairly simple DIY but culligan may have proprietary parts that you have to get through them.

More than likely need to either change a filter or look at the tank.

That would be my first guess.

~egon

Agree with the above. Most home systems are simple and should have "industry standard" parts. Can't say if this is true for Culligan or not.

Used to work for Culligan in college. Tank probably needs replacing and the RO membrane and carbon filters I'm sure are overdue for replacing. But the issue you are having is from the tank. If I had come to a home with that problem all I would've done is replace the tank provided the water test showed low amounts of solubles.

In CS, Culligan rents out a full system for a monthly charge and that includes replacing the whole system with new filters/membrane I think once a year or so. Or you can buy the system for a flat fee that was pretty expensive; barely anybody did that. I would just call Culligan and tell them your address and see if the system is a rental and what the history of it is / what options they can give you for fixing it. I'd say it's worth it, as RO systems filter municipal water to the same quality you'd get in bottled water.

Probably plugged/spent filters. Do you have a softening system leading into your house or is your RO doing all the work?

I don't have a Culligan, but I would check the tank first. The tank holds about 2 gallons worth of water. The tank should be full and heavy if you are only getting 6 oz. Take the tank outside and use an air pump to empty the tank and pump up the pressure to 7-8 psi. The tank should be light when you re-install.