There are a couple of things you need to really look at before you go spending alot of money on diagnostics on this horse.
Feeding: Exactly how much senior feed are you giving?
Unfortunately people have this idea that senior feed is really good for putting on weight but they don't understand how it has to be fed to do that. It helps put weight on senior horses because it is more easily digestible and can be used to replace hay that a senior horse likely can't really digest. Senior feed is NOT calorie dense like grain based concentrates. It is a high fiber, forage based feed that must be fed in pretty large quantities to really improve weight. If you are feeding a 1000 lb horse something like Equine Senior he needs 13 lbs per day if he's not getting any hay. And if he is getting 12 lbs of hay per day he still would need 6-9 lbs of senior feed per day just for maintenance. That's not to increase weight--just to maintain what you've got. People often try to feed complete senior feeds like they are sweet feed and they just aren't the same. So, it really may be that you simply aren't feeding enough for weight gain.
For a 6 year old horse that has no other health problems (including needing floating), free choice good quality forage and a good protein/vitamin/mineral supplement to go along with the hay will generally maintain weight. If a bit of extra calories are need then the addition of fat to the diet can work wonders. 1-2 cups a day of vegetable oil (or people use sunflower seeds, flax, etc) for fat can greatly increase the digestible energy in the diet and is more easily digestible and less likely to cause health issues than a high grain diet like sweet feeds. Or you can use any number of grain based feeds that are out there on the market and that are designed to provide energy for working horses---these can be used to put weight on healthy horse. Just be sure to read the label and feed whatever you are using according to the directions---they aren't all the same.
Teeth: When were they last checked? Poor alignment, uneven wear and missing teeth can really impact how much a horse can get out of anything you feed. If her teeth haven't been checked in the last year, they need to be.
Deworming: You say she has been on a regular deworming schedule every 6 weeks, but what exactly are you using? The old rotational plans aren't good---especially now that resistance to commonly used dewormers is so widespread. It's fairly commonn for people to be thinking that they are effectively killing parasites in their horses when they aren't.
In the parasite control game, deworming more often isn't always better. Choosing the right product at the right time and deworming only as often as really necessary are what you want to strive for.
Have you had fecal egg counts done on your horses? Do you pick up feces out of your pasture daily or every other day? What are the weather conditions typically like in the winter and summer?
Licensed Veterinary Technician