Without seeing it, it's difficult to diagnose precisely. Light strikes with DWs, particularly if the mainsprings have been changed, are not unusual (for a number of reasons.) No strikes, though, are a bit odder.
I have seen several guns where the sideplate was not fitted correctly, leaving a small "ledge" at the junction of the sideplate and frame up near the rear sight. In one case, the hammer (which was very loose on its pin - another DW characteristic) wobbled sufficiently that it caught on the little ledge, which slowed it so much that it did not impact the primer hard enough to leave a mark.
Of course, another cause could be severe timing problems; this usually manifests itself on one particular chamber, which isn't getting rotated far enough to come under the firing pin. Check carefully; if it happens on the same chamber consistently, it's most likely timing. (Check the other primers for off-center hits, as DWs that are out of time usually have some sign on the other chambers as well.)
Of course, grit or dirt in the firing pin channel can have this effect, though it is not at all common.
Hope this helps!
