I had the same in Madrid. Tried to get a taxi for 15 minutes. After that time I had already walked to the next taxi rank and got in the first taxi there. Mytaxi always showed cabs close by but wasn't successful. Wasn't even a busy day, the taxi rank was more than full.
As much as I don't like uber, surge pricing can be a really helpful feature that I would like to have with cabs as well.
Or even just let cab companies compete on price, maybe with some limits. When they could compete on price in Budapest it's true that a lot of tourists got ripped off, but it was awesome if you could afford one of the more expensive companies. Once they introduced uniform pricing the better companies got flooded with calls and availability dropped... and tourists still get ripped off, but probably less often and usually when coming from the airport.
As much as I don't like uber, surge pricing can be a really helpful feature that I would like to have with cabs as well.