We recently had a gemara that discussed forcing the new month to be a malay or choser. Someone in the shiur noted that it should be easy to force the month to be full. If beis din closes for the day and is not in session on purpose then we can make it a full month. We simply don't take adus hachodesh on that day. If we wish to make it chaser, however, and the moon is not seen on the 28th day because of clouds how can we possibly make it chaser?
See Rosh Hashanah 20a, where there is a Machlokes whether it is better to tell the Edim to withhold their testimony, or whether it is better to have two Edim say they saw even though they didn't. It is only Edus in as much that it can be followed with the declaration "Mekudash," for Beis Din has the power to make Rosh Chodesh on either day.