Watering is all relative. "Once a week" can be "I spray it with the hose as I walk by once a week" which isn't going to do a thing. Or "I stand and soak the tree for a few minutes once a week" still not nearly enough. Or "the sprinkler system waters the lawn once a week for half an hour and it gets some water". Maybe, maybe not enough.
How much water you give it once a week makes all the difference. An hour's soaking with sprinklers would be a good amount in cool weather, but if you have a spell of weather in the 90's in the middle of summer, all the plants need more. Maybe twice a week.
A tree or large shrub needs deep watering - someone mentioned soaker hoses and that would be a great way to water this tree/shrub. Be very sure to water out at least as far as the branches reach, because the tree will have roots out that far and more.
But if it hasn't been growing the way you think it should, I would definitely start out in the spring with some good, deep waterings once growth has begun, then as the weather heats up, going through the summer, check the soil under the mulch every few weeks during hot weather to make sure the water is getting down deep enough that the roots are getting it.
Btw, now is a very good time to move the hostas, too. They will be dormant soon, (if they aren't already?) and as long as you plant them nicely, and take a good root ball along when you move them they will be fine and have better roots established by spring. Oh, and mulch them well for the winter, too. That will make sure they don't heave with the freeze/thaw cycles, and also help retain moisture in the root area.
Digging around the root area of your Wahoo in the spring doesn't sound like a very good idea to me, either. Get those plants outa there, and get the thick mulch down around that tree NOW! Really!
