I apologize: I'm gong to try to answer this even though I have not been taught zen.
I read a collection of zen koans a long time ago and I thought that they were cool (i.e. like an attractive puzzle) but kind of slippery (difficult to grasp or solve, difficult to get inside a koan and understand it).
In summary my answer is going to be that, maybe koans are good but they're not enough?
I say that because, since the time when I read those koans, I had more experience of life, and I learned (read, thought about, tried to learn some basic vocabulary of) a bit more Buddhist dharma. So now perhaps there's a bit of Buddhist dharma in me (or "in my understanding / vocabulary / mind").
Now when I read a koan, instead of trying to get into it, instead of finding myself being able to get into it, my experience is that it gets into me. When I attempting to parse (make sense of) the koan's words, they connect with bits of dharma that are already in me. One koan is like a ball with several spikes, that sticks inside me, and each spike connects to or activates a different bit of dharma, and joins them together or, I don't know what (because I'm not practising that).
If you really want to pass this barrier, you should feel like drinking a hot iron ball that you can neither swallow nor spit out.
For example, "what is your original face" makes me think of what I've learned about what Buddhist dharma says about "self", gets me thinking about that, and wondering "if I understood this koan (and understood Buddhist dharma about 'self') then how would that affect 'me', would it change my behaviour or outlook or the way people see me, etc."
So again, in summary: maybe they (koans) are good but not enough.
If you're learning (or being taught) Zen, I've no idea what other Buddhist Dharma (sources of dharma, suttas, teachers, lectures maybe, vocabulary) you're exposed to; perhaps the koan combines with these somehow?