Seriously, I feel this in my soul. My starter was a total dud for weeks, just a sad, flat pancake. I finally stuck it in the oven with just the light on, and it was like a different creature by morning. Turns out my kitchen is just too cold and it needed that cozy spot. I was so close to dumping it in the compost.
Ugh, I read somewhere that the temperature in your kitchen can totally wreck a starter's mood. If it's too cold, it just sits there like a lazy lump. Try putting it in the microwave with the light on, just don't turn it on! That pet rock idea from @clairetaylor is looking better every day. This thing is more dramatic than my last roommate.
Used to fight my starter every winter until I tried proofing it in the microwave with just the light on. That warm little cave turned a lazy lump into a bubbly beast overnight. Maybe these things just need a better spa day than we realize? Clairetaylor's pet rock is still a solid backup plan though.