It sounds like you are not engaging the center "lip" in the center of the seat. Standing on the left side of the bike, with your right hand, pull up on the front of the seat. With your right hand, push down on the center of the seat to engage that center lip and then the front of the seat will lock down when you let go of the front of the seat. If you set the seat down on the bike and look from the left side under the center of the seat, you will see how that lip of the seat engages onto the bike.
Easy once you see how it all engages.
+1
Need to "bend" the middle of the seat down, then push the front down. Otherwise, you go above the center lock.
MP