Gaming EXP cheat works one time, then it doesn't

[Kamika]

I was playing Pokemon Diamond on my TTDS and I activated the exp cheat to level my pokemon faster. But the exp cheat doesn't always work. I was told to hold the R button at the end of the battle and that works, but not all the time. Does this cheat have something to do with timing or is my rom being buggy?

 

[Exbaddude]

Try removing it and then add it again?

 

[elixirdream]

try hold it a few seconds before end of battle?