If you're feeling shy about asking professors for recommentation letters, remember that it's normal to feel a bit anxious and unsure on how to go about it. Know that most professors do not mind at all
(a little secret: most are quite happy and flattered to be asked).
When a professor says no, it's usually because they don't have the time to write or don't feel they know you well enough, not because they don't feel you deserve it. If they say no, there's no problem, just thank them and ask someone else.