Hi BL,
Ink spot in a strelitzia is caused by all the things you stated in your post, if they are in a sunny spot, they can get burnt in hot sun,wind. To shallow,poor drainage will also will not help.Being a more tropical plant,it my not like your winters either. Position is very important for strelitzia, shaded in summer heat, warm sunny position in cooler parts. Deep good soil as they are a big rooted clumping plant and need plenty of room to move/ grow bigger. Good luck. Peter r

Hello there the problem you describe is very typical of a condition called Bacterial leaf stripe.... It is commonly seen in strelitzia if they are waterlogged , under fed or stressed.. try a seasol drench every 10 days and spray the plant with a mix of Anti rot and copper oxy chloride @ 2 gram / L