The reason for this could be that you used archive download-sw/leave-old-sw in the download option. The /leave-old-sw option keeps the old software version after a download. When you enter reload, this only reloads the active switch. This fails because the switch as a stack expects all models in the stack to have the same version of the image. As a result, the active switch is placed in a disable state and the standby switch is in the active state.
Use the archive copy-sw privileged EXEC command on the stack master to copy the running image from the Flash memory on one stack member to the Flash memory on one or more other stack members. It copies the software image from an existing stack member to the one with incompatible software. That switch automatically reloads and joins the stack as a fully functioning member.