Remote control of Samsung Android devices doesn't work.

In the past have been able to remote control a Pendo 7 tablet and computershowever never a Samsung Galaxy device. I have tried: Galaxy Tab 8.9, Galaxy Tab 2 10.1, Galaxy Y Young, and Galaxy S5 (all from Samsung) - none work. After installing the Teamviewer Host including the Add-On for Samsung devices on the Galaxy device and registering then the remote Galaxy device appears in the Computers and Contacts list as expected but the two remote control options in the drop-down list are both greyed out and remote connection is not possible.

Any solutions? This has been a really useful app to support my aged mum but her Smartphone use is suffering as she finds the new devices strange and frustrating - hence the hoped for remote support by Teamviewer :-).