We have never laid wood. Just finished laying about 350 sq ft of a hickory engineered wood, and noticed there is quite a big buldge in one area and a smaller buldge in another area. They are not very visually noticable, but it when you walk on the area you can feel a bit of a hill.
We stapled it to plywood subfloor that we thought was level, left 1/2" expansion gaps. Let the wood acclimate for a couple days, even though we heard you don't necessarily have to do that with engineered wood.
Any thoughts and suggestions would be greatly appreciated, we have a thousand more square feet to go and are not sure what to do

If there's deflection in that area that means something is loose - nopt fastened correctly. If no deflection then the substrate was not flat.

Check with a six foot straight-edge. The flatness goal is to get within one-eighth of an inch tolerance in six feet or three sixtieths of an inch in ten feet. If you have a bump in a traffic area the finish will get scuffed over time on the bump because we don't lift our feet as when marching, normally.
General contractors only need to be within a quarter of an inch.