Title pretty much says it all. Items that share a common border don’t combine.
Example:
Create two rectangles, one wide, one tall. Move bottom left corner of tall rectangle to top left corner of wide rectangle (should be ‘L’ shaped).
Boolean union does nothing, should combine both item one ‘L’ shaped geometry.
Try making the tall rectangle 1 mm taller, set grid to 1 mm, then over lap the top left of the wide rectangle with the bottom left of the tall rectangle by 1 mm using snap to grid. Then select and boolean. Alignment should be retained.
Just add another small rectangle (or any shape that will be entirely contained in the new combined part) that slightly overlaps both parts that are just touching. Select all 3 and do a union!