The charge type on of the second merc calldown is set to global and not unit, hence the action targeting it will not work as that action is for charges on units only.
Additionally the compound starts out at maximum charge count. More charges cannot be added until at least 1 charge is depleted and even then it will still max out at the maximum charge count.
One can set the maximum charge count to something large, eg 999, while keeping the starting count low to allow a lot of charges to be added and stored up. One might also be able to dynamically modify maximum charge amount for a player using catalog actions to set it to a new value, however last time I checked each catalog modification performed by the catalog actions resulted in a permanent memory leak.