Besides checking with your pediatrician (some doctors prefer to wait until a child is 3, but I disagree), you can contact the Infants and Toddler program office for your county and request your daughter be evaluated. It is a free service provided to evaluate children for speech and learning disabilities for early intervention. I have 2 friends who are used this service, and their boys are now receiving free speech therapy to help them with their delayed speech issues (both women already have 2 or 3 older children).
My granddaughter was recently evaluated, and we hope to hear her results any day now. She just turned 3, but I have known since she was 18 months that there was an issue which wasn't solely related to the fact that her parents weren't working with her very much or well.
This website helps parents to to their own evaluation to see if they should pursue further help. I found it to be an excellent resource of information. Although some of the references may have you thinking it's in regards to autism, it is not. I highly recommend you take a look: http://www.firstsigns.org/.
The Infant Toddler program (sometimes called Child Find or Well Child) is a fully funded, national program that is available to everyone. You can look on this website for your local office or contact the HQ for the number. They will help you through the process of evaluation (@45 minutes) and, if necessary, further help: http://www.family-networks.org/infants.cfm .
If your daughter is accepted, they will not only help her directly, but give you techniques on how to work with her. One of my friends who uses this service (it was for her fourth child), says she was surprised at what she learned, especially after having raised 3 good talkers. They gave her tips that were very obvious when explained, but are often overlooked (like getting down on eye level when speaking with your child).
Your daughter's frustration in communication is a clear sign she is having difficulty expressing her needs, and I would recommend pursuing the program. If it turns out to be nothing, then you will have peace of mind that your daughter is just developing on her own terms (albeit a frustrating course for you). If it does turn out to be something, then you'll be glad you acted on it earlier rather than later.
Good luck!