...but lots available on same flight when booking as new ticket.

I was booked on a flight SMF-IAH in mid- September, in F, on an A fare (paid). Tried to change the SMF-IAH leg to SAN-IAH, and there were no first class seats available on any flights the same date. Tried to get F the following day, and the next, then the preceding 3 days. Every attempt showed no F seats available SAN-IAH on any flight on any day, in any fare class.

Then I tried to book a new flight SAN-IAH on my original, preferred date; and 7 out of 10 seats were available. I grabbed what I wanted (again, on a paid A fare), and cancelled the original SMF-IAH flight.

My question is, since seats were available on my desired SAN-IAH flight, and since the fare classes were the same, and all fares in the itinerary were fully changeable and refundable, why did my attempt to change the SMF-IAH leg to SAN-IAH show no seats available, when they were clearly available when I booked a new flight? TIA for any insight.