Endnote can display two citations or zero but not one. Deletes citations when transferring from one style to another.

I had some text like this in one style, citing four references:

This is my text.(1-4)

When I switched to another style (American Journal of Physics), it changed to superscripts (correctly) and deleted two of my references from the bibliography:

This is my text.^1,2

Where I am using the ^ to indicate a superscript.

I tried to add back in one of the the missing references, and I got:

This is my text.^1,2,3   ^3

In other words, a second ^3 appeared although I had only dragged in the reference once.  When I tried to delete the second ^3" Endnote deleted the first ^3 as well.  In other words, Endnote will not let me put in just one copy of this reference; it must be zero or two.  Can anyone suggest how to fix this?