Simple manual autofill not working

I have an alphanumeric field which I would like to autofill - but it only copies. Can it be done?

For example:
Value: Q2.1
I want to autofill Q2.1.a, Q2.1.b, Q2.1.c, ...

I've tried clicking and dragging with Q2.1.a and Q2.1.b highlighted but it just repeats the value (Q2.1.a, Q2.1.b, Q2.1.a, Q2.1.b,..)
When I autofill Q2.1 it increments correctly: Q2.1, Q2.2, Q2.3, ...

How can I autofill the tertiary outline level?

Any help would be appreciated!
Thanks

You could put this formula in your first cell:

="Q2.1."&CHAR(ROWS($1:1)+96)

and then copy it down as required.

Hope this helps.

Pete

Thank you Pete. I will use that formula.

I have multiple "Q" values so was hoping the autofill would be able to handle it.

I replaced the alpha with a number and it autofilled correctly - then I tried just a, b and it was copying again... I thought autofill also worked on alpha values...