It's also possible you just got a dud yeast packet. It's pretty rare, but I've had it happen once... in 140-some batches... I think mine was a packet of EC-1118. Repitched another packet after a few days of no activity, it took off immediately and was fine.
But I agree with the others, it might be the high SG, anytime I go over 1.125 I like to make an acclimated starter of at least 1/5 the volume of the batch, I rehydrate the yeast as per directions, then add an equal amount of must as there was rehydration water, wait till it looks and smells like it's doing something, then add must to double the volume again, repeat every 30 min to an hour or two until I've got a good volume, then in it goes.
"The main ingredient needed is 'time' followed closely by 'patience'." - The Bishop 2013
"Good grief! If someone wanted to murder you, all they would have to do is ship you a 55 gallon barrel of honey and watch you document working yourself to death!" - Vance G
"When you consider that laziness and procrastination are the fundamentals of great mead, it is a miracle that the mazer cup happens." Medsen Fey, 2014