Bottom seems nicely filled. Top may be passably filled where it is supported by the infill, but droopy everywhere else. The sagging may be why the lines don't touch. Or else it may actually be underextruding a bit. Main problem to me is the perimeters are not perfectly lined up. I would drop the extruder temp a bit (190º-200º?) which may help with both the top sagging and the perimeters staying put.

Do you have a fan for cooling the part? Probably not necessary for the cube, but if you are going to do a lot of PLA it could be very useful.

Edit: BTW, nice self diagnosing/fixing so far.