Tried all those sites on my pihole and no issues at all.
I am on version 4.0 (one upgrade down I think) and running on a zero w.
It would be worth checking the pihole log in the admin screen and see if anything shows up - something may be swamping the server with lots of errors.
You should see something like:
Feb 10 18:20:33 dnsmasq: 4778 192.168.1.182/56775 query[A] www.cos.edu
Feb 10 18:20:33 dnsmasq: 4778 192.168.1.182/56775 forwarded www.cos.edu
I found my router kept sending the same request to the DNS despite being told it did not exist. In the end I had to move DHCP off the router and onto the pihole itself.
Need Pi spray - these things are breeding in my house...