I work in real estate and hopefully can be of some help.
What times are you contacting her? For example, if you are contacting her on a Sunday or at 9:30 PM and are not hearing from her until the next day, I think that is totally reasonable. Additionally, agents are often at appointments showing homes for hours at a time, and might have a "phones off" policy while with clients.
My suggestion is to ask her what her policy is. What days and hours does she work? When can you expect a response? Does she have other team members that can assist you if she is not available?
Be up front about your concern, because it is true that a slow response time can lead to the loss of a home you love, and telling her your concern gives her the chance to resolve this issue.
If you have not signed paperwork committing to work with her exclusively, you could also switch realtors.
I do want to throw out there that I personally think it would be unreasonable for anyone to expect their agent to work 7 days a week and to respond the same day always....everyone needs a day off, time with their family, etc. (I am just saying this because I do see that many people expect their realtors to always always be available, even if is 11:00 PM on Christmas Eve, lol) but your agent should also be very clear to you about what her working hours are.