Does the "cap" look like
this? Or is it just a dust cap? An un-terminated splitter port could cause trouble on one or two channels
A new cable between the antenna and one set will offer the most helpful diagnostic information at this point.
If I were offering suggestions for a all new antenna system, an amplifier would not be on the list of needed items. You should be able to receive enough power with a roof mounted antenna, to feed eight sets.