If you are under level 5 you might be getting hit by the spam filters they recently implemented. From the latest (1/31) patch notes:
Rogean: Accounts with characters under level 5 will now be subject to the following chat limits across all public channels:
- 2 Messages per 10 seconds.
- Messages must be under 80 characters.
- Messages must contain 70% Lowercase alphabet.
Rogean: All characters are subject to a one message per half-second limitation across all public channels.
These particular rules came about because there were folks creating throwaway characters and mercilessly spamming populated zones. If your messages don't violate the above rules, then I am not quite certain what might be the cause. I personally have never seen the "message not sent" warnings that you mentioned, although I admit to not being the chattiest person around, and I type slow.