I think it's possible your credit score is being adversely impacted by your utilization ratio if you only have 1 or 2 cards. We don't have a mortgage or any other debt, but we do have an untapped HELOC and a pile of credit cards, so our available credit is a very large number. At any given moment, we owe current month-to-date gas and grocery expenditures, which we pay off at the end of the month, but only represents a tiny number relative to available credit. Our current FICO is 856.
I open new cards for the sign-on bonuses and then often never use them again. Occasionally one is a winner though, like the Amex we opened last year that pays 3% on groceries and 2% on gas with no annual fee. That has become our primary card most of the time, although right now we're working on an initial spend to get a $200 bonus on another new card.