Hello, Vee,
Fish Tales seems to have the trough switches numbered from right to left, so the switch closest to the shooter lane is No.1. Why couldn't it mention left, centre or right?. The few games with four switches have to identify between 'left center' and 'right centre'.
But serving out two balls is often a sign of a trough switch 'hanging up'; after serving one ball correctly, the furthest left switch previously Closed should change to Open, if not (or not quickly enough*) the feeder serves again. Of course, this couldn't occur with only one ball in the trough (though the feeder might operate), but it may be affected by whether a ball is held in the reel lock - with three balls circulating Left Trough (No.3) would be critical, with two it would be Center, No.2, though 3 playing up wouldn't help in that case either. Have a good look at the trough switches, maybe try each of them in Single Switch while moving one, two or three balls through the trough.
* while I'm afraid it's not applicable to Fish Tales, Dr. Who has a special Self-Check routine, including a specific 'Trough Test' which checks for 'gummy' switches; after establishing that the solenoids work, it's emptied while watching the timing of the trough switches