When using Elastic or User Layers, they do not get attached to the end-user VM when the user logs in.  Checking C:\ProgramData\Unidesk\Logs\Ulayersvc.log, you see an error like this:

2018-07-19 13:02:25,804 ERROR 16 CompositingManager: Caught exception: 'Castle.MicroKernel.ComponentActivator.ComponentActivatorException: ComponentActivator: could not instantiate Unidesk.Guest.Core.Services.VirtualDisks.VirtualDisk ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ComponentModel.Win32Exception: A virtual disk support provider for the specified file was not found

The actual Windows error here (which you can also see if you manually attach the VHD through DiskPart or DiskMgmt) is "A virtual disk support provider for the specified file was not found".

There may be multiple causes for this.  The error really just means that Windows is being blocked by policy from allowing the disk to be attached.  One identified cause is a specific GPO:

Computer Configuration/Administrative Templates/System/Device Installation/Device Installation Restrictions - Prevent installation of devices not described by other policy settings. 

If you disable that setting, you should be able to mount the VHD manually using DiskMgmt or DiskPart, and Elastic Layers should start attaching properly at logon.  Be careful, of course, to find where that GPO is set.  If it's in your domain policies, then the setting might be captured in the Platform Layer, and not get cleared by the updated GPO before a user logs in.