You need to research Amiga hard hacks, especially Agnus ones thoroughly before attempting them.
You have to know all the niggles about the version of motherboard you have (relevant to the hard hack).
You have to know the hard hack inside out.
If it involves replacing chips (like an Agnus hack) you need to know all the different variants and numbers etc. and be absolutely sure that this is the right one for you.
If you tell us which A500 motherboard you have, which region (PAL/NTSC) which agnus it was fitted with and which agnus you tried to replace it with and maybe we can help?