I suppose it is possible that the iPad 3 could be the source of the issue.
Still interested to find out why some images will load from LRM > PSMix and not others.
Checking with the team to see if the iPad 3 could be causing the grief.
One solution we could attempt is if you direct messaged me your login credentials, I could log into PSMix and your LR accounts and try selecting images on my iPad 4. (device comparison testing)
If you'd like to try that with me, shoot me a message, and I'll get at it.
Will send you a message back when I have finished testing, so that you can change your password if you'd like.
Regards,
Pete