People usually do not feel comfortable to say no when they are been asked to write a recommendation letter as they do not like to imply that they have negative views about your work. However, most decent people would try to avoid writing you a letter if they feel they cannot write a good letter. So when you ask for a letter always try to leave as much room for a polite refusal as possible. Therefore, phrase your request as a question, something like: if you feel you know my work well enough, would you be able to write me a letter of recommendation? If the answer is not yes or if they express too much hesitation, you probably better off asking someone else.