Recommendation letter for application to CS graduate program from professors in math? I graduated from Dept A with a master degree, and then pursued a PhD in Dept B (which is applied math). But I left the PhD program in B this fall. I am now applying to some CS master programs (including some professional master programs and non-professional ones).

My research in Dept A used some computer programming and machine learning methods, although A isn't CS. In Dept B, however, most of my experiences were taking courses and doing TA, and very little research in stochastic processes.

I asked my previous research advisor in Dept A for a letter of recommendation. He politely say no, because we didn't have academic interaction in the last three years after I joined Dept B. Also I guess that the fact that I didn't finish PhD in Dept B may  be another reason. Note that he recommended me when I applied to PhD program in Dept B, and he knew little about computer programming. 

So I find some professors in Dept B, whom I took probability and stochastic processes courses from  or was a TA for. I was wondering if their recommendation letter will help my application to a CS master program  (including some professional master programs and non-professional ones), given that these are two different areas and my experience with CS was mostly from my master study in Dept A?

Does recommendation from my previous research advisor in Dept A really not help much, because our collaboration was too old, compared to recommendation from the professors in Dept B?

What if I am applying to PhD Programs in Statistics, instead of CS master programs?
When applying to graduate school (either PhD or Masters level), it helps to have letters from any past research supervisors or graduate programs. The fact that your advisor at Dept A refused to write you a letter means that you probably do not want that letter. Letters from people who only know you from class or teaching are not as useful as research letters, but they can still be helpful.