Climate (A/C) Mode Control

It seems the 'Mode' on my 1999 Buick Century A/C climate control is 'stuck' on blend or bi-level. Half the air goes to the floor, half goes to the dash vents, no matter where I set the mode control. I have checked some fuses, but not sure which to check. Also have changed the cabin air filter, that was surely dirty and clogged. Any insight before I head to my mechanic will be appreciated. Thanks in advance.