"Some have shown too much defensiveness and have not shown enough focus on digging into, and addressing, the problems.
"Also I am concerned that some do not properly ... understand the need to communicate to the wounded and their families that we have no higher priority than their care."
Gates said a new permanent head of the medical centre would be announced later on Friday.
An investigation by The Washington Post found that soldiers convalescing from war wounds at Walter Reed were subjected to a morass of bureaucratic red tape and housed in a decrepit building.
Criticism of hospital
Joseph Lieberman, an independent senator who supports the Democrats in congress, said the administration and Congress must hold the Pentagon and army accountable for veterans' treatment.
"The White House and Congress have an urgent obligation now to fix the neglect at Walter Reed and the longer-term issues that affect our wounded veterans," he said in a radio address for Saturday, but released on Friday.
George Bush, the US president, has ordered a review of practices of the hospital and promsed to make any necessary changes.
"This is unacceptable to me, it is unacceptable to our country and it's not going to continue," Bush said in his weekly radio address, taped on Friday and released before its usual Saturday morning delivery.