Richard:

What happens is this: every time you insert an object in a wall, SH3D (internally) begins a new wall segment. That means that it also re-starts the texture. It is this effect that you have noticed.

If it is important that the texture is seamless, you can make a tailor-made texture for that wall. See this manual .

Another solution would be to insert the window at the exact start of a new texture. Maybe using a close-up view of the textured wall will allow you to place the window (or door) on the exact right place.

Hans