Some teachers get so many requests for college recommendation letters that they simply can't accept them all. You can often make yourself stand out just by being polite.
Ask your teacher at an appropriate time during the day, and be sure to ask as far in advance as possible. Provide your teacher with anything they may need, like an email address or letter template, work samples, resume, and anything else that can be helpful. When you make it easy on your teacher, they'll be more likely to say yes.