I recently had a walk-in shower installed in the space previously occupied by a cast-iron bathtub. The new shower drains extremely slow, and I find myself standing in an inch or so of water during showering. A drain-cleaning company ran a cleaning snake through the floor drain all the way to the main sewer and said it was clean with no blockage. Their theory was that it was not vented properly, because of its location past the vent stack.
What would be causing this new shower not to drain properly and quickly? More importantly, what would be a viable solution?