A few questions. I ask just to put all cards on the table.
1. Are these speed signs on your static object tab or track side object tab? (I've oopsed this one once)
2. Are you sure these are scripted signs? (There are a few that are just for show)
3. Do you happen to have other splines of any kind overlayed on the tracks? (Catenaries, powerlines, fences, 3rd rails, etc...)
4. Have you tried other speed signs to troubleshoot this issue? (This one may answer #2)
5. (Dumb one, but I did say all cards.) Are they facing the correct direction?