St Andrew Square is (or should be) a really convenient interchange point between tram or bus and bike (eg, if you'd rather not slog up Leith Walk on a heavy hire bike), and handy for the eastern part of Princes St, George St and St James, so it's not surprising that it is very popular = often has no bikes available.
I have suggested to them that they should add docks on every corner of St Andrew Sq in recognition of this fact, but, as usual, have been ignored. They ask for people to suggest hire points, but then rarely seem to actually act on them, it's very frustrating.
Here's the hire point suggestion form again (which is hard enough to find a link to in the first place), if anyone is so minded:
https://docs.google.com/forms/d/e/1FAIpQLSft70Zbl-4Z5-gszn9BReuPcxxTl2NQCDkoNU5UjvCxRzupaA/viewform