Usually this happens when your logistic or tactical slots are at fully capacity, and you just ordered and upgrade for those slots. The building will get placed in this case, but the constructors will make no progress on them until the planet upgrade is completed.

Its also possible the Vasari Loyalist titan building a phase stabalizer could fill up your tactical slots right as you're about to build something, preventing any more work on it until tactical slots are available.

Actually, you do.

If you queue up the logistic upgrades on the planet, you can immediately set down structures to fill that expanded capacity.  They will not begin building however, until the logistic upgrades complete.