Hi Tracey! Oh wow, I am not sure why that would be happening. Ours are very flat. We didn’t have that issue at all and we also did this same exact technique in our master bath remodel. I am thinking it’s a different type of plywood maybe? Maybe too much glue (moisture, etc)? All I can offer is maybe use a different type of glue or maybe quite a bit less? I’m so sorry I’m not more help!

LOVE your shiplap walls.
We have been shiplapping our ceilings with 1/4 inch luan plywood. We’ve run into issues with the edges of not only, the full sheets, but our cut strips rippling on the edges. We’ve had to make braces to put against them while we wait for the glue to dry. They are also nailed. How did you avoid the edges of your planks rippling? They look very flat in your pictures.

Yes, I believe it was similar in price but still much cheaper than using the pre-made shiplap. You could look for and use a less expensive plywood. (they have some that are 1/8 instead of 1/4, for example, and also different finishes, etc) Other than that, I don’t really have any suggestions, sorry!

No, I didn’t but that would look great too!

NICE RESULTS…. DID YOU CONSIDER A LIGHT USE OF “DRY BRUSHING” WITH A SLIGHTLY DARKER COLOR (OR HAVE YOU SEEN) TO GIVE IT A MORE “WEATHERED” LOOK?

We are actually running into that in our beach condo that we are renovating. For us, we have just decided to create the “broken” more rustic look for the longer than 8 feet wall we are putting it on.(pieced together like a wood floor)
Another option for you is applying it vertically instead. If you have 8 foot ceilings or large trim up top and below, you can apply it vertically instead. Hope that helps!