The best food to try is earthworms. You can also try slugs, slaters (just make sure the garden you get them from hasn't had any garden pesticides used). Try baitshops for earthworms. You may be able to get live blackworms as well from the petshop.
Otherwise frozen bloodworm. Mealworms should only be used occasionally and the heads should be chopped off, as they can harm your axie. Beef liver should also be an "occasional" food not a staple, and any fat or tendons should be trimmed so its lean. Do not use feeder fish, they carry parasites, excrete a lot and can/will nibble your axie's gills.
How long have you had your tank up and running? How long have you had your axolotl? Are there any other tank companions? What substrate are you using? What chemicals, if any have you added to the tankwater?
Did you cycle the tank: see:
http://www.caudata.org/cc/articles/cyclingEDK.shtml
You need to test your tankwater to rule out anything that may be effecting your axie. The most important levels you need to test for when there may be a problem, or your tank is cycling, are ammonia, nitrite and nitrate.
If you don't have the relevant test kits, take a sample of tankwater to your petshop. Ask them to test for the 3 mentioned. Most importantly ASK them to write the results/figures down against each one and give it back to you, don't accept it's ok - you need the figures so you can tell if anythings wrong. .
Once that's done, if they suggest that you buy a bottle or two of something that will fix your tankwater. DON'T buy it. The only chemical you need for your tankwater is one that is used to treat tapwater to remove the chlorine and chloramines from the tapwater.
Pop back on here once you have the results and someone will help. Basically, if ammonia and/or nitrite levels are above 0, then you need to do a 20-30% waterchange of your tankwater. This will bring any toxins down.