Within the past 2 weeks or so I've been having trouble sending texts (plain texts), I'm using the stock messaging app, when I go to text someone it'll say "sending" and it'll stay that way for anywhere between 5 - 20 minutes before actually sending the text. I've also had trouble receiving texts, sometimes receiving them hours after my friends have texted me.
What kind of coverage are you showing on your phone? (How many bars of service) Usually when I have seen that (with Sprint anyway) it was due to crappy coverage in my current location. Next time it happens try moving around or going outside and see if that has any effect.
I am using a LG G3, my zip code is 11426. I've had the issue regardless of where I am, I live in Queens NY, I've been to the city (Manhattan), long island, and other places in Queens and continuously experience this issue.