I will assume you did the ask correctly by making clear what you needed and when you needed it by. T are doing you a big favor so you get to go back once to ask again. The best way is something like this... I appreacite that you are taking the time from your busy schedule to write a letter of recommendation on my behalf. There are some dealines that are coming fast. I on't want to bother you, but would it be possible for you to complete the letter by (put in the date)? If the persona does not do the letter after the second ask you should move on. In fact you should ask more people. If you require 3 letters of recommendation ask 5 people to write them. This way you are covered. Some people cannot say no to someone's face so t say yes and don't do what t promised. Some have good intentions about doing you this favor, but are too busy to get it done. Best of Luck.
When to ask for a letter from previous colleagues If a student who has previously worked with my staff needs your recommendation letter, that is very different to asking if they need to have a letter of recommendation from a previous staff (or other students' letters). When you ask such a question, it is important to first ask the relevant teacher to explain how they have previously worked with the student. Asking after a teacher's previous relationship with a student can be a very awkward conversation if you expect the teacher, in the future, to come back to visit the student. You may need to ask the teacher's assistant to help bring the person back to your class. Asking for such letters would only happen as a final resort. If a student is new in my class, and my previous colleagues don’t know how to work with them, I try to keep them.