Did you ever sit with them at night to look for light leaks.
Something as simple as a few minutes of >1 Lux of light during the dark 12, will reset their chemical timer.
Too much nitrogen will inhibit flowering as well.

No leaks? No flashes?
Flush them with plain PH'ed water and try 11:13 for a week.

As a last resort they can be stressed into flowering, but best to do it gently if possible.

Aloha,
Weezard