you are right! she wasn't able to read at beginning but after while she read his mind,
First case can be that Anya was too far away from him to read his mind.
Second case can be that He was probably not having any thoughts at all (maybe he is used-to it or he practiced) but after talking to Damian he had some thoughts, maybe thats why he avoids interacting with Damian.
Yep if demetrius knows then he obviously was trying to block out unnecessary thought, weird theory i have is that crowley family, loid think anya learning classical language is connected with her upbringing and crowley also said he wants to talk with anya regarding that and also crowley name appeared again as magical crowley, a fortune teller, maybe just weird coincidence and author is running out of names
i think people are forgetting that anya DID/COULD read demetrius' mind (see end of chp.93):
--damien intercepts demetrius to congratulate him on his stellas (demetrius' thoughts: 'I don't understand him.')
--damien asks if their father said anything about his own stella win (demetrius goes quiet, turns away to leave saying how would he know, they aren't in regular contact; thoughts with a close-up on his eyes: 'I don't understand father...')
--emile and ewen speak to demetrius as he is walking past, introducing themselves in case he forgot (he's briefly quiet, says 'oh.'; thoughts: 'I don't understand people.')
--demetrius continues walking away (thoughts: 'Enough of this. Who cares? Pondering it is a waste of time.')
with what we've learned further from melinda and now potentially donovan, i think demetrius' thoughts here have more theorizing potential than previously. especially since melinda said demetrius had also noticed/believes donovan can read minds. (or at the very least, is strange/off)
anyway! mostly i just wanted to remind people as some commenters seemed to have forgotten anya DID read demetrius' mind, it just wasn't immediate because there (seemingly) wasn't a thought to read yet. or she was indeed just not close enough, like she wondered.