What Scrypt coin are you mining? When did you start seeing the dip of your payouts?
Pool shouldn't really affect your payments. Assuming they actually mine and picking pools actually has an effect, then you might get short term variance where some payments will be smaller, but it should be balanced out with larger payments on some days. As time passes though, your payments should keep reducing as mining difficulty is expected to keep increasing. If you look at Litecoin (the first Scrypt coin IIRC), the difficulty has been going up.