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
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"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
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