Why can't I switch my pokemon out?

Question

I'm playing Pokemon Diamond at the most, trying to level up my level 1 to level 100. I'm doing so by defeating the elite4 over and over again - my problem is that when I kill one of their pokemon, I don't get an option to either "Change Pokemon" or "Keep battling" like I used to be able to do. Is this because I've killed the elite4 so many times?

Answer 1

same as the three last said

you need more than one pokemon in party

Answer 2

Press X and go to options and make sure that Battle Style is set to SHIFT.

Answer 3

go to your options menu and  turn battle style from set to shift. You must of accidentally changed it. nothing to be ashamed of bud.  

Answer 4

press x and go to options then change the battle style to shift

if it still doesn't work you must have a glitch or something