It was believed that Lumia 650, which is presumably the last Lumia that Microsoft could release in 2016, would debut on February 1, but according to a report by WindowsCentral, that plan has changed, and the company has pushed the unveiling to mid-February.
While the reason is not yet known, it appears that the company came across some software or firmware issues with the phone.
http://www.windowscentral.com/lumia-650-announcement-delayed-until-mid-february