Should I still ask this person to be my letter writer?

[taacc]

Hello! This is kind of a strange situation and I was hoping to get some other people's opinions about what might be going on.
I am applying to PhD a second time this coming fall, and I was hoping to re-use one of previous rec letters. Between the first application cycle and this upcoming cycle, I graduated undergrad and did more research. The letter I'm re-using is from someone who was essentially my undergraduate advisor and has known me for a few years, so I believe this would be a really strong letter to include again.
However, I'm having a hard time getting in touch with the aforementioned undergraduate advisor. In the past year, I sent a few emails hoping to catch up and to tie up some loose ends with work that I did during undergrad. They replied to my first email and suggested we catch up sometime, but have since gone silent on my follow up emails (which have spanned over half a year). I have not yet formally asked them to be my letter writer again in emails, but I'm wondering if I should still ask this person to be my letter writer again given the silence. Am I being paranoid/is this professor just busy, or is this a hint that they are no longer enthusiastic about being my letter writer/keeping in touch? I'm pretty stumped because I thought I left undergrad on a high note, and I can't think of anything egregious that I've done that would explain the silence. At this point I'm just praying they'll reply as I don't want to bother them further with my emails (and the later request to be my letter writer again). Additionally, I fear that if they do reply, there's a risk that the letter will be lukewarm as I don't know what caused the silence. Should I politely ask if they still feel comfortable being a letter writer, or stop pursuing this? What would you do? Thanks!