I noticed that anytime I plug in a switching transformer anywhere in the house it blocks X10 signals. Some block when plugged in and off, others block when on. Is there a mod that can be done to the transformers?
There's nothing you can do to the transformers themselves - as others have suggested you can use a filter.
What voltage/current rating are the transformers that are giving you trouble? Part Number?
I tested 3 that are X-10 friendly. I'll try to find the PN for one that came from Mouser as well as another from another supplier - I may have published them in my article on X-10 noise. This one from Smart Fun
was OK...
However, there's no guarantee that one you buy today will have the same internals. Still, it might be helpful to start a list of those known to be friendly/unfriendly.
EDIT: Well, d'uh - I wrote a short article about SMPS and X-10, listing both those referenced above.