You're not pressing hard enough. Do you have any young snakes? Smaller snakes are easier to pop with very little pressure. The big ones need a lot of pressure to pop. And if you can't see anything, that means you being too light.
Personally, I hate probing. Its much too invasive for my liking. Ever since that one guy here pushed a probe through his snake's hemipene, it got me all paranoid.
And what can be more accurate than seeing actual genitalia sticking up at you. Lol
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