David may have more to say but you can play with this
=DATE(YEAR(A1),1+MONTH(A1),DAY(A1))

If it also fails to work with anything in column A you should highlight
the problem cell and examine with menu item Format/cells - it's probably
not interpreted as a Date by Excel.

Is it possible to have a cell say "b1" read from a
cell "A1" and automatically change to the following month.
So If say "A1" was January, I want "b1" to be february but
I need it to be calcuated by a function. To date I have
just been adding 31 days to each cell in column a which
has worked. i.e "b1" formula reads = a1+31 etc... I am
using David McRichies "Insertandfillformulas" but for some
reason it can't read column A this formula and things
aren't working !.