I had a similar problem recently trying to book a space from Reading to Bristol on a Sunday. Going out on Saturday was no problem, but nothing available on Sunday. Tried Temple Meads, Parkway, from Reading, Didcot, Swindon, Chippenham, or stopping short, but nothing whatsoever. Same when I went in to the station. Recent threads in various places have made me think that there might be a trend now to book bike places on a 'just in case' basis, which is a bit... annoying.