Hugs, Scarlet. I'm sorry. It is really difficult when they offer to provide something, then don't follow through. It seems like she's promising more than she can provide, which isn't fair to you. I don't know what the answer could be. Possibly for her to check in at some point within 24 hours? I don't know.
I know for me, Dr. T pretty much always replies to emails in the mornings, generally between 7 and 9 am (and he says this, that he will reply to client emails each morning). Recently, there was a time when I emailed the night before and hadn't heard anything back by around 11 am the next morning. I realized maybe it wasn't clear I wanted a reply, so sent an email clarifying that and saying that if it was that he was busy, a reply the next morning was fine, to just let me know. He responded within an hour, apologizing that he'd been to busy, but would reply that night. I knew not to expect that. He didn't reply that night, though did reply the next morning.
I'd have preferred if he'd just said "I'll get back to you by tomorrow" instead of saying he'd respond that evening. I did ultimately talk to him about this yesterday, saying that if he knew he couldn't get back to me in the normal timeline, could he possibly just send a very brief email or text, like, "Busy, sorry, will reply when I can." Just to acknowledge it. He said he could do that. I know that might not work in the situation with L, if it's a case where she might intend to text in the evening but falls asleep, say. But maybe you can figure something else out? That she'd then send you a "good morning" text, say? I don't know.