When not to zealot rush?

I have just discovered the wonders of proxy zealot rushing. I setup a pylon and 2-3 gateways somewhere near his base, spam zealots and attack. In silver league, this is effective about 70% of the time.

When I'm spotted and encounter heavy defenses, I can often switch to some other strategy with relative ease - since I control the access to my opponents base, I can usually fast expand and go air while he is blocked from his natural.

I'm not saying this is 100% victory - if he spots me early and/or manages a good defense, he can out-race me to the air - but still, it's usually quite effective. Besides the bordedom of repeating one strategy, what reason not to use this strategy all the time?

(Note - it is a bit less effective against Terran because they usually wall in quickly)

Proxy gating will tend to fail against those that scout it and respond appropriately. The response will depend on the race. Part of the problem is that your main is left undefended and you are spending your minerals on zealots instead of economy. If the enemy scouts your proxy gates and gets a few units to your main while defending with cannons/spines/bunkers/more t1 units than you, you will tend to lose.

The interesting thing here though is that there is no answer for "when to or not to zealot rush" as you can't know what your opponent is planning for an opening. You sort of have to hope you win immediately or can recover well enough from a failed rush that you don't subsequently lose. The advantage here is that you are setting the pace of the game and putting the opponent on their back foot. Good players will tend to retaliate after defending the rush netting you a loss due to your undefended main while less experienced or nervous players will try and turtle (which is not advantageous because it gives you map control).

Proxy will tend to beat:

Fast Expands as the opponent tends to build less combat units and spends more on economy.

Early Tech rushes (trying to tech to voids/thors) as more is being spent on tech than combat units

Proxy will tend to fail to:

In-main 2 gate rush or 3 racks builds or early spawning pools with spine crawlers. As the defender will be making attacking units in base and advancing their economy.