How to deal with reference who routinely sends recommendations late, or not at all

[mockturtle]

I'm on great terms with my undergrad thesis advisor, but she has a history of dropping the ball with recommendations. I have no reason to think it's personal, and her graduate students have all attested to experiencing the same thing. Over the years she's sent more letters of mine late than on time, or else completely neglects to send them at all, even after ample time to prepare, regular reminders (both emailed and in-person), and promises that she would get to them "by the end of the day" or "definitely before leaving the lab tonight", etc. I of course understand that she's an extremely busy woman, but this is honestly perplexing. Aside from this one issue she was a great advisor and never seemed to have a problem with deadlines in general, or keeping up with emails in general, or remembering things in general - it's just references. Having worked in her lab for 2 years and after completing my thesis under her she's definitely the most important person on my CV, and would be a bad person to leave out of my reference list.

I graduated last spring, moved 1400 miles away, and am currently applying for internships..... and one of my contacts is telling me he hasn't been able to reach her (not even for a letter, just for a chat). I can feel history repeating itself. I reached out to her recently to ask whether she was okay with being contacted, and she confirmed once again that she was happy to serve as a reference. I even mentioned this person by name as someone who would likely be emailing her in the near future. I'm not sure what else I could have done better, or what to do now.

Is there any way to explain this situation delicately without seeming like I'm putting my advisor on blast, or affirm that she agreed to serve as a reference without seeming like I'm being defensive? Any action I can take in the future to avert this? I'm so worried about having it seem like she's unwilling to recommend me because of the quality of my work, when it's just chronic late-LOR-itis. Thanks so much.

[Concordia]

I've got a weirder problem-- my most important reference was my master's dissertation advisor.  He got me into his place of work, but hasn't yet filed the [same] letter for my other two choices (for which there is still time, thankfully).  Flattering, but annoying.