Not sure if Coates.tom is referring to his Airport Time Capsule or his MacBook Pro System Preference Settings dropping the IPv6 setup? I am having a similar issue with only one of my MacBook Pros. Our Airport Extreme's IPv6 is enabled and incoming is blocked.
Have activated IPv6 for your MacBook Pro under advanced settings in the Network System Preference panel. The 2nd tab TCP/IP is where you activate IPv6. After activating it, I have no problems communicating with my Airport Extreme. BUT every time I reboot my MBP it turns off my IPv6. If I try to access my Airport Extreme from my MBP 2014 it shows that it is not part of my network, till I re-activate my MBP 2014 IPv6 under Network System Preference panel. I am able to access the Airport Extreme from our other MBP 2010 which does not revert back to default IPv6 settings.
Have followed all of the trouble shooting tips I can find. Rebooted everything and made sure my settings mirror our other Macs on our net. But only my 2014 MBP is dropping the IPv6 when I reboot it. Even ran the IPV6 test mentioned in the above post after activating it with perfect results. Oh and we are running El Capitan on all our Macs, too.
Only my MBP mid 2014 is reverting to default IPv6 off settings, none of our older Macs in the house are having any issues with IPv6. We run several iPads, iPhones, a MBP 2010, and a iMac 2010 on the net. Anyone else having this issue and if so anyone figured out how to correct it?