The last dozen of my PvZ I try to push early with 2-4-6 zealots. Sometimes opponent has not much defense and I can kill queen, few drones and the most important - force him to build more lings.
It looks like I got a little bit better on the ladder, so this strategy stopped to work: usually or quite often opponent has spore crawler or roaches.
With crawler - that is ok, I pulled back get more stalkers/sentries and keep putting pressure.
But versus roaches I don't really have an answer: Immortals will be few minutes later... and instead of pushing opponent I'm being pushed. I can still defend (mothership core, few photon cannons, etc.) until I get immortals, but I lose map control..
How would you advise to continue the mid game once opponent get roaches?