Regretfully, SMS messenging (texting) is a service that uses spare bandwidth on the voice communication channels (IIRC). Due to its nature, it is not guaranteed to be delivered, or how quickly it will be delivered if it is. I have had some messages from my wife take several hours to get to me, while others she sent later got to me just fine. Some she has sent never arrived at all. In the past month, I've had a few cases (2-4) where messages she sent to me didn't arrive for 15-30 minutes. If there are tower issues in your area, that could explain the issues you're having as the remaining towers are working "overtime" to pick up for the towers that have problems.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.