Can you check the logs, perhaps you got some kind of a error in your IDLE connection (just search "IDLE" word in the logs). In case of any error with that connection account's sync scheduling might be moved to 15 mins interval until first successful sync.
[1294] Feb 15 15:43:00 [Debug] IMAP : IDLE: An IMAP IDLE related error occurred. [1294] Feb 15 15:43:30 [Debug] IMAP : IDLE: IMAP IDLE error recovery trying to establish IDLE state now. [1294] Feb 15 16:40:03 [Debug] IMAP : IDLE: IMAP IDLE error recovery detected that the primary connection is busy. Retrying to establish IDLE state in 30 seconds. [1294] Feb 15 16:40:33 [Debug] IMAP : IDLE: IMAP IDLE error recovery detected that the primary connection is busy. Retrying to establish IDLE state in 60 seconds. [1294] Feb 15 17:04:58 [Debug] IMAP : IDLE: An IMAP IDLE related error occurred. [1294] Feb 15 17:05:28 [Debug] IMAP : IDLE: IMAP IDLE error recovery trying to establish IDLE state now.
[1299] Feb 13 00:29:48 [Debug] ../../../lib/SignOn/authsessionimpl.cpp 173 checkConnection [1299] Feb 13 00:29:48 [Error] no requests to be canceled