In the following example, page number entries in the index are not 
displayed when a blank line is inserted in the text, but do appear 
without that blank. This happens with both the current 20140316 
standalone beta and with TL2013.

(I noticed this constructing MWEs for other index register problems. It 
is likely not a problem in real life, but may provide insight into other 
issues.)

    \starttext
    Why do the entry page numbers disappear when a blank line follows
    this line, and reappear when the blank line is removed? (Current
    beta and TL2013.)

    \index{AA}
    \placeindex
    \stoptext

--
Rik