Make sure these things are in place, and it should work:
1) - Have the secretregiontrigger in place, and being both sufficiently below ground and above what ever should be hidden. 
  2) - The trigger needs to know it's locked. Put "
LockSecretRegion(TRIGGERGUID_SecretRegionTrigger_000_117f85c7-0f28-4636-a01b-5c3ddb614853);" in the 
INIT section. 
  3) - Make the hidden region unlock after an event of your choice, in my example case after clicking the "Rabbit Goes Boom Lever". Yes, I like exploding rabbits.
Add this under 
THEN in 
KB section.
"
UnlockSecretRegion(TRIGGERGUID_SecretRegionTrigger_000_117f85c7-0f28-4636-a01b-5c3ddb614853);"  
  4) - And likely the missing cause in your case, open 
Level > Level Data and make sure "
Hide Shroud" is set to '
False'. 
  Then it should work.