When did you last clean your filter? If the filter is very dirty then the chlorine will be used up trying to oxidize the gunk. I had this problem close to closing last fall.

I changed my filter cartridges about 3 weeks ago-- they were totally mucked up with pollen that had fallen in the previous month.

We clean our filter cartridges every month. We cleaned them and the salt cell two weeks ago.
Maybe I'm not running my filter long enough to compensate for chlorine loss?
I'm running it on low speed for 4 hours and 3 hours on high. My manual says 8 hours at least, so maybe I need to bump up the time. I know the salt cell is working because after bumping up the percentage to 75% and running it for an addition 2 hours after the normal time, my chlorine was close to 5ppm by this afternoon. I lowered it back down to 60% and I've set my time to run about 2 hours longer and I'll see how it does for the next week.
I could call my PB, but it's been two months and I don't like to call him unless I really think there is a problem. I should know this stuff by now.

This morning I went out and I'm down to 1ppm again. I bumped it back up to 75% and will run it for 9 hours and see what it looks like tomorrow. My skimmer has a few ants in it, but that's it.
So as long as I'm showing high levels after the SWG runs, then that should prove that the problem is not with my generator. I'm not sure how to check it on low speed.
I just turned it to low speed on my remote, went to the diagnostics menu and it is reading my salt level and voltage. I'm assuming it's working if it's reading those items?
It runs on low speed from 1am until 6am. Then my high speed kicks on at 8am until 11am. When I go out at 8am, my chlorine reading is low. I would think after running 5 hours even on low speed that I would have chlorine. After running it on high speed for 4 hours yesterday, I had plenty of chlorine.

I found out what was lowering my chlorine. I use something called ScaleTec to keep the scale from building up on my pool. It is not recommended to use the full dose in the summer because it can lower your chlorine and cause algae, but even though I have only been using the maintenance dose, it still lowers my chlorine. I found that if I shock the pool the day after I put the ScaleTec in, it will maintain the chlorine level.
Oh well, I learn something new every day.

I am considering transition to a Salt Water Generator. I currently use chlorine in a vinyl lined inground pool approx 20,000 gal. Any recommendations on what brand or type to use? The prices and features are all over the place!

I use the Goldline system. It's all automated. We just have to clean the salt cell every few months. My system even reminds me to do it. I think it's Hayward brand? I like it, but I don't know how they work with Vinyl pools. Maybe someone who knows could chime in with an opinion? I know Leslies Pool sells the same system I have, but I don't know how much it is. Lesliespool.com might have a price.

Its around a $999. I'm surprised you have to clean your cell after a few months...

We only had to clean ours about once per year before....I wonder why there is so much more maintainance required.

Well, it's not that we necessarily have to. Our system reminds us every few months to do it, but instead of just clearing it out, we just go ahead and pull out the salt cell and give it a good rinsing as a preventative measure. My PB told me that if we do it every few months, we are less likely to have problems with it later. Our water here is very hard water too, so it is a good idea to prevent build-up on the cell.