The usual pattern is a stack that grew by addition. A managed partner here, an extra SSP there, a header bidding wrapper nobody has reviewed in two years, floors set once and forgotten, and a direct rate card that no longer matches what the market pays.
The result is predictable: high fill, low yield, demand partners bidding against each other for the same impression at prices the publisher never set, and reporting that describes the outcome without explaining it.
None of that is fixed by adding another partner. It is fixed by deciding what the inventory is, what it is worth, who is allowed to buy it and on what terms, then configuring the stack to enforce those decisions.