Intersect tool - Repeat pattern tiles not lining up

Hi-

I've been making a repeat pattern by tiling a design:
Working in a 4in square for the tiles.

I group my objects, place a no-outline no-fill 4in square on top of that design. Then use the intersect tool to cut out a clean 4in square tile.
With a 4x4in document grid, (snap to grid + snap to object) I ctr C + ctr V then slide the new tiles to 'click' into place along the grid  - horizontally and vertically - until I have the desired size of my repeat pattern for printing.

It worked perfectly for 2 weeks, the object outlines of each new tile fitting perfectly with its neighbours.

And now they don't, they're out by a tiny gap on almost all the object out lines on all sides.

I've tested my earlier patterns within their original docs and they all work, but the newest documents don't, and I'm baffled as to why not. To the best of my knowledge, I haven't changed anything. I lie, I added a fill to the objects but initially that didn't prove a problem either.

Any ideas? I'm a bit desperate now as my print deadline is looming.

Thanks!