Ok. I have some possible cenario's why it doesn't work as intended.
1) The ability editor has two abilities having the same name. Ensnare in orc category, and ensnare in naga category. Check so that you picked the right Ensnare, and remove the requirements for both.
2) Are your example above an action with variables or "already-placed units"? Maybe you did something wrong, if you have the variables...
3) The Ensnare target must be visible for the Ensnare caster. It can't cast Ensnare on an unit within the fog of war.
4) Bridgand has the "hide" ability, allowing him to stealth when its night (you know I suppose). The Ensnare unit can't cast it on a hidden unit (however if the stealthed unit is hidden, but detected you can.)
Hope this is the answer on your problem : ) ?
/Regards