Not sure what you mean when you say you have checked the battery and charger and they are OK. I think the BDI setting is 1 to 9 in the features or performance menu. 5 is the factory setting. When people say they are not getting any run time, it can mean a lot of things. What is their expectation? Six hours continuous until it goes into lift interupt is about the norm I have had people tell me the battery is not holding up and find out the meter went down 3 bars in 2 hours which is normal.

I would try some of these things until you nail down the problem.
1. Check all the cells with a hydrometer and battery fully charged.
2. Check all the cell voltages.
3 Make sure every cell has water. Single point watering systems can have a bad filler or injector and cause a problem
4. Check the battery under a load, such as max lift at relief pressure. I would be looking for about 33 volts charged and no less than 28 or 29 volts fully discharged. There is no spec in a book of this. It is just what I have found from experience.
5. Check the brake air gap..010 to.040 is the spec ( off the top of my head). Shoot for.015-.020.
6 Swap the battery with another truck.
7. Have the operator use a different charger.
8 Have the customer put it on equalize charge every night for a week and keep the water up. Have had this help run time many times. Are they doing a weekly equalize charge or are the chargers set up to do this automatically every week?
9. Make sure they are not short charging the battery. Sometimes operators will charge at lunch, see a full charge, and have the meter drop real fast in the first 15 minutes.

As far as checking the batteries, I am not a battery tech, but I think you are looking for no more than 10 points (.010) difference in specific gravity and no more than.05 volts difference in cell voltages.