XF 1.5 Like button missing for some members

I have a strange issue where three of my members are unable to see the "like" button.
I have talked them through the usual procedures (try another browser/clear cache/try another theme etc.) but they simply can't see them.
I have checked their usergroups/permissions and all looks good.
This has happened since I upgraded the server to PHP 5.6
Any ideas?
 
An update.
I have been able to log into the forums as a couple of the members (they kindly gave me permission and passwords)
The situation is this: four of the 350+ members see the like button on all posts posted more than three days ago, but if there is an OLD thread but with a new reply they DO see the like button on all of the posts in that thread.
All the other members can see the like button normally on all posts.
I am so confused. I have re-checked permissions, using diferent themes and I can't for the life of me see what I'm missing.
HELP
 
There's nothing in the code that would apply date limits or anything like that to the ability to like a post. I could only guess that perhaps an add-on is interfering, so please confirm the issue with all add-ons disabled. If simply disabling the add-ons doesn't resolve it, please then go to <url>/install/ and choose the option to rebuild the master data to see if that helps.
 
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