One of the problems could be that the weight on wheels microswitch may have not operated, thus the aircraft still thinks it is airborne and BETA range cannot be selected, therefore ground idle and reverse thrust cannot be selected. A worse problem would be the other way round - ie the aircraft is airborne and the weight on wheels has not moved in the other direction and it would then be possible to select BETA range in the air and could lead to the loss of control of the aircraft as BETA range allows the prop angle to go beyond the airborne range and into reverse.