I am building kitchen drawers. I have 24 inch cabinet bases and bought 21 inch blum Blumotion soft close slides. I built the sides and backs of the drawers out of 1/2 inch Baltic birch. I used a Whiteside drawer bit to join the pieces. I made the sides 21 inches, which I thought were within the parameters of these slides, but the sides seem about 1/2 inch too short to lock in to my locking devices. Any thoughts? Did I make the drawers 1/2 inch to long?