One of my professors didn't respond to my Letter of Recommendation email reminder. What should I do?

Technically, this is for summer internship applications and not college/grad school but more than likely I’ll run into the same issue when applying for those as well.

I asked 2 professors for an LoR. One of them I had a close connection with and know that I can rely on her. The other is the one I have issue with now. I had him for a chemistry class and in my last semester, I worked with him in his lab for a research assignment I took on for the professor that I was close with. During the semester when I needed to meet up with him, he would respond very quickly, even quicker than my other professor. And working with him, he was very friendly to me and we got along pretty well and had a few great conversations that didn’t deal with my research. So because of this, I asked him in December if he would be willing to write me a LoR and he said because I had a class and worked with him, that yes he would be willing to. He told me to remind him via email whenever I’m ready.

I filled out 4 summer internship applications but he needs to only turn in LoR’s for 3. The 4th one only required one LoR which is being taken care of by the other professor. They are all due January 31st. I know that as soon as I turn in my applications, those schools notify my professors via email that a LoR is needed from them. Unless they went to the spam folder, I know he’s gotten them. But I reminded both professors anyway on January 19 via email. The first professor responded to me on the 21st letting me know that it was being taken care of. But this second professor did not respond at all and it’s basically the 26th now and no response. I’m getting worried because the due date is getting close, but I don’t want to feel like I’m harassing him with another email.

Is it ok to send him another email? A few things I’ve been reading online said calling him or meeting him in person is best but I’m not so sure. Should I look for another professor? I do have another one in mind but it’s cutting it real close to the deadline. I’m a bit frustrated right now…

I would pick up the phone and call, or better yet, stop by and ask personally. Email just doesn’t seem to be an effective way to reach him.