With the integration between Preno and channel managers, reservations are made by purchasing a number of nights from a room type. Rooms aren't sold individually, so they're aggregated into the number of available rooms per night. You may end up with the situation where a booking comes in for a length of stay that's too long to immediately fit, but the availability for that room type matches. Preno always tries to place bookings into the first available room of a room type.
Your channel manager will display this as your availability:
A reservation could come in for the period of the 1st of January to the 6th of January, but your grid may look like this:
When this situation arises, Preno is unable to reject the booking from your channel manager. The space is there for the booking, but it requires some shuffling to make it fit. Preno will place the booking in the first room of your room type, and display a warning about the double booking. You'll then need to shuffle your existing bookings around in order to make space.
The availability was always there, but existing bookings were preventing an extended length of time. Periodically, you should reshuffle your reservations around with the intention of maximising your available rooms for reservation.
There are a few ways to minimise this happening - taking one or two rooms offline would give you extra space for moving bookings around.