Careful placement helps, but it also depends on the design itself. There should be something that connects them, or seems to connect them. Mirroring every other one in the border gives that continuous impression too.

I digitised a flower to demonstrate what I mean.

Picture 1 shows a design repeated. As you can see that works well.
The design in picture 2 looks nice but the end seems to need something.
That design is mirrored in picture 3, and the second one placed a bit lower. But you would need editing software to make placement easy.
Repeating that combo as in picture 4 gives a nice continuous effect.
And as I could not stop playing I mirrored the combo - picture 5 - and am stitching that on a small finger towel now.