Do you know why the radio doesn't always have work out available?
The TV is much more predictable in its requirements - that is, for there to be some space available in front of the box so your Sim can lie down. The skill gain is also faster, which may be why it's favoured by the Skillinator
The radio on the other hand is much less reliable. Sometimes the workout interaction, even when available, will drop from the queue as soon as you pick it