CO's undersized premium cabins are really one of my biggest issues with this airline. I'm new to CO and have already had two experiences where I wanted to buy an F ticket but couldn't because every flight for an entire day is sold out in F. The most recent example is a flight I have this coming Tuesday between EWR and SFO. I have a paid ticket in the F cabin and now may need to make a change to a flight earlier in the day. It's still more than 48 hours before the flight and every EWR-SFO for the entire day is showing zeroed out in F. Wednesday's flights have only a single seat. If I do end up needing to change, I will end up just refunding my CO fare and buying an F seat on another non-stop, probably from JFK. I'd really like to try to give CO a chance to earn my business but the ridiculously underzied F cabins are making it very difficult.