Weak and mushy seam!

I am fairly new to baking, but have got to the point where I can reasonably competently shape a batard, up until the final stitching. I am using 40% WW and 80% hydration. So, having folded the dough "trifold" from a rectangle, I roll it up and all appears good - a puffy dough, fairly tight skinned over the top, and ready for the banneton. I put it upside-down in the banneton ready to stitch for some final tension... However, every time, the dough on the bottom, now facing me, is just like a weak and mushy pulp. Trying to stitch it causes little tension, and I end up just pulling around increasingly mushy and messy dough. When I watch online demonstrations, what they are handling when they stitch always seems different. In all other aspects though, my dough and shaping of it appear largely the same. Does anyone have any ideas please? Thank you.