Hey Ed,
Heed what Geno said.
I'd normally say check the nut when tuning issues rear their head, but since you are only going flat, I have to surmise it's likely due to insufficient stretching.
You didn't mention whether this is after a recent string change. Strings will eventually "settle in" once strung up, but getting to that stasis comes much faster if you have properly stretched the strings first. If not, then every time you play the guitar, the strings haven't "finished stretching out" so to speak, and will absolutely drift flat because playing the guitar is akin to weakly stretching the strings. This "stretching out" can be slow and irregular, which then can be mistaken for a physical issue with the guitar when, in fact, it's just the strings needing to find that point of stasis. So yeah, stretch em out. I have always found the plain strings need considerable stretching out, whereas the wounds need very little to remain stable.
Alternative thought: do you use case your guitar in a soft bag? I say this because all my guits are rock-solid stable in keeping tune except for when I use my gigbags. I'm sure it's the tuning keys getting slightly nudged when going to/fro, which obviously will not happen using a hard case,
Edward