ELISA - POD problem

[Misfit]

Hello,
	We are using an ELISA system which now gives low readings for POD
substrate (POD). Our ELISA protocol consists of adding AP and POD,
washing unbound Ab, adding the AP substrate, then washing away that
substrate and then adding the POD substrate. 
	Besides changing reagents (which we have tried and has no effect)
are there any other things we should check and test?

Thanks in advance,

K


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