Finally figured out why the baseboards were failing

I had a similar issue when I installed wainscoting in the dining room. I thought the problem was the height of the molding itself, but once I got down on my knees and checked the floor with a level, I realized the hardwood was warped. There were these tiny humps and dips that made the base of the trim look crooked even after cutting the end pieces perfectly. It was frustrating because you spend so much time getting the cuts right and then the foundation ruins the whole look. What worked for me was using some self-leveling mortar under the first few rows of baseboard. I bought a 10lb bag of quick setting type from Home Depot and carefully mixed it and poured it in a thin layer where the floor dips the most. This allowed me to correct the initial slope before laying the trim over it. It wasn't a perfect fix for the entire room, but it smoothed out the biggest problem areas near the walls.

For the rest of the gaps between the trim and the wall, I used a small amount of high quality wood glue and then filled the remaining minor voids with a spackling paste mix, letting it cure completely before painting. It took way more time than I expected but the finished line looks incredibly clean now.

I had a diferent issue come up when I was installing trim in a hallway last spring. Everyone assumed the problem was the baseboard molding itself, but it turned out to be the drywall installation right behind it. I was running a piece of decorative 3-inch profile trim, and instead of sitting flush, it had this subtle rotational twist, almost like it was pulling away from the wall at the corners. After taking the trim off, I noticed the drywall joint where the two sections met wasn't properly butted against the framing. It looked like the paper tape had been applied over a slight offset. The whole wall section was slightly out of square, even though the studs felt solid to the touch. Trying to shim the baseboard against a slightly bowed wall surface was just making the gap look uneven. What fixed it was cutting out the offending drywall section and resetting the joint flush to the stud. It was a pain, but fixing the wall preparation first meant the final piece of molding fit perfectly without any of that lifting or twisting.

I ran into this on a kitchen remodel where the old subfloor had shifted after a plumbing repair. The problem wasn't just dips for the baseboard, but localized flexing which caused issues with all the trim. I found that securing the trim directly to the stud at 16 inches intervals rather than relying on wall anchors prevented almost all the lifting I saw in your post. We used 75 gauge nails and kept the spacing exactly 16 inches on center. For minor surface variation, shimming helps, but for structural stability, you have to anchor through the drywall into the solid framing.

I've seen this problem before, and it's not always the floor that's the culprit. Sometimes the wall itself is throwing off the alignment. If you don't have the drywall properly sealed or if you've got moisture migrating through the studs, your trim is gonna act up. I had a run of baseboard that kept peeling away from the wall near the bottom, and it turned out the wall cavity had a wierd moisture issue. It's always smart to check the wall prep before you start fixing the floor.