My mobile updates won't turn on :(

I've checked the "sleep" restrictions in my device settings, and they're definitely not on, but every time I try to turn on my mobile updates (I turn them off when I'm at work) by texting "On" to Twitter, I never get a response, and I'm not getting any Tweets. This used to work fine for me, but it doesn't anymore.

Does anyone have any suggestions, because Twitter isn't letting me reset my device settings either.