The Featured Slots are based on the current date plus one month. So for dates beyond (and in non-leap years) including the 28th of January, you will not be able to book featured slots until February 1.
For example - January 30th + 1 Month = February 30th. There is no such date, so until date + 1 month = a valid date, no further future slots can be bought.
Accordingly, on February 1 slots for March 1 will become available.