There are at least a dozen possibilities, so without more information we can't be much help.
Can you iMessage to other iPhone users?
Do you have iMessage enabled in Settings/Messages?
Does the other person have iMessage enabled in Settings/Messages?
If it is enabled, is it enabled for both WiFi and cellular data?
Are you messaging to the correct number or email, and does the other person have the Messages settings checked for the address you are sending to?
I'm having same problem with sending to 3 people from a particular WiFi Network. For two people the iMessage i write is sending, but stops just before Delivery and then says not delivered. For the third person it immediately sends as text message.
Answered to questions above.
Can I send to others... Intermittent. Usually no, but then at some point a lot of iMessages comes at once.
Settings have been enabled.
Also other persons settings/Messages are enabling iMessage.
Enabled for WiFi + non roaming cellular ( at my end it should mean only wifi as I am in roaming area with wifi available).
Numbers are all enabled as we have used them a lot of times, an no changes or upgrades this month.
So: it seems to me to be a WiFi issue here, but the odd thing is that it sometimes lets iMessages through.
e-mail and browsing works fine.