It may be that they didn't feel comfortable writing a letter on your behalf and didn't want to say so. It could also be that they are legitimately busy, and it's more time out of an already packed day. In any case, it's better to have an enthusiastic letter writer than a lukewarm one. Have you completed observation hours in a variety of settings? If not, go into your next with a solid game plan to get a LOR.
One of my letter writers was a PT I hadn't known previously. When I first met with her, I explained that I would need an LOR at the end of my time there, and asked if she would be comfortable writing one if we developed a good rapport. She stated she would, and then I worked hard to put my best foot forward over the next few months - dressing professionally, being quick and responsive when asked to help out, asking questions when appropriate, and doing follow up research on what I saw. Observation is not the most exciting thing, but I think it's crucial to remain engaged, think critically, and engage with your PT as much as possible (without being annoying) so they get a sense of who you are when it comes time to write that letter.