Cannot get TapAndDragGesture to work on X300 trackpad

[pelago]

Hello. I am trying to resurrect an X300, and have installed Ubuntu Mint 17.04 on it. It's mostly working fine so far, except for one thing:

I cannot get the feature on the trackpad where you double-tap and keep your finger on the trackpad in order to drag or select text. It works fine in Windows.

From Googling, this feature is called TapAndDragGesture in synclient, but that is already set to "1".

Any ideas?