Check to make sure you are not running out of IP space. The symptom you are describing:
1. Some client work fine some do not at the same time.
2. Wlan is Open (WebAuth is only for Authorization to use the network, not security)
3. Some device used to work and not does not.
Check Monitor>Active clients and see if there are any client with IP address in the upper range of your IP pool, if you have a 192.168.1.x/24 and you see a client with 192.168.1.250 or higher you have probably have this issue.
You can do a few things to mitigate this issue:
1. Put a simple PSK key there so casual device do not grab and hold an IP
2. Increase your IP pool, though even this can be used up if the lease time is high and you are in an area where there lots of people.
3. Shorten the lease time on the DHCP server so the IP can be re-used, in public space may be 15-30 minutes, depending on the flow of device grabbing IP, intended or casual device.