Dumb question but I'll ask it anyhow .. when you're dosing Mg, are you dosing it to bring it back to 1400 or are you dosing the amounts suggested on the bottle? Oh wait, IS there a bottle? What are you dosing for Mg?
Here's my take. Mg is an underappreciated element by the powers that be within the hobby. Commercially available Mg to dose is like taking a peashooter to a shootout where the other gunslingers are carrying Uzi's (Ok my metaphor sucks, but I'm too lazy to think of something else. I use the "it's like draining a lake with an eyedropper" metaphor far too often and it's time to shake things up a bit). I would buy a $30 bottle of Mg solution from the LFS and then end up dumping the entire bottle into my water change water to make it 1400. So now I just buy 20-30 lbs. of MgCl crystals from LittleSilverMax because it's a far more cost effective solution, costs me maybe $80 or whatever but I end up with enough to fill a 5g salt bucket which lasts 6 to 12 months. I end up adding 225ml of it (almost a full cup) to my makeup water to make 1400 and when I dose my tanks I found it surprising how much of it I would need to raise Mg by 100. I forget how much exactly (I write stuff like that down so that I don't need to remember it, consequently, I don't remember what it is when I'm writing a post like this. Sorry.
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So if you're using a commercially available product to dose Mg and just following the instructions of "one capful per X gallons", my bet is that this is just not enough to maintain Mg. It wasn't for me anyhow and I'd be surprised if that truly works for anyone.
Over time Mg will deplete just like Ca and Alk will. Think I've heard the addage that the depletion rate is roughly the same per week as it is for Ca or Alk per day.