We recently upgraded from 7.1 > 7.5 and ever since then it seems that some pools do not have the option in HTML access to have multiple monitors. The settings in the pools that work and don't work have the have pool settings as well. I have tested this on Chrome Firefox and IE. I have attached pictures.
Try performing a restart of the environment to clear any ADLDS (ADAM) replication issues. I've seen and heard reports on multiple occasions of odd issues after an upgrade that this will resolve.
Multi monitor is only supported on Chrome browser for HTML Access, please check your pool setting and make sure "Max number of monitors" is set 2 or above, and make sure your VMs are powered off then power on after the setting is changed.