Using Lydia as an example, do the following:
(1) take everything that you've given her back (temporarily)
(2) Tell her "It's time for us to part ways" best to do this where she would be anyway at whatever home you have that she returns to
(3) With her in the screen, bring up the console and click on her so that her Actor Value number is showing
(4) Type "Setav health" and take note of the number it gives
(5) Type "disable"
(6) Type "enable" (you may have to get out of the console and go to where she respawns)
(7) When she reappears, open the console, and highlight her (so her Actor Value number shows)
(8) Type "setav Health" This should now be a larger health number, if you've levelled up at least once since she joined you
There's a lot of unnecessary steps in that procedure. Simply dismissing, disabling and enabling her, then rehiring her (or any other follower) will do the trick. I do this every five levels with Erik to keep his stats mostly current (wait any longer and he gets too squishy; do it every level and I get annoyed by it).
If you want to check on a follower's health you should use "
getAV health" as well. It will report their current health and a computed base, which is what their health should be for their current level, without any buffs or debuffs added in.
(You can also use the getAV console command to check on other stats as well, like magicka, stamina, and various skills. When checking for stuck follower stats though, health is probably the best one to check, especially for tank followers. You'll notice the biggest difference in actual vs. computed base there if you haven't "unstuck" their stats in a while.)
Also note that, unless you're on PC and have a mod that removes follower level caps, when you hit a follower's level cap they will stop leveling and their stats will stop increasing. If you have a follower who's gotten "stuck" at a certain level--typically 40 or 50 for most followers--that's why. They've hit their level cap. Without mods, J'zargo is the only follower who will level up to 81 with you.
And finally...if you're playing on XBox 360/PS3, since you don't have access to the console, the same method of unsticking follower stats can be accomplished via Wabbajack. Remove your follower's gear, take them to an isolated area, save, dismiss, and whack them with the Wabbajack. (Save beforehand just in case they turn into a Dremora as opposed to, say, a chicken. Isolated area is to avoid aggro'ing any NPCs nearby, who will interpret this as a hostile act/a crime.) Give them a quick punch to knock them back to their normal selves, then grab them, rehire them, and give them their stuff back.
Slightly trickier than the console method, since Wabbajacking is quite unpredictable in its results, but it will unstick their stats.