Try Dan’s recommendation and only inflate to 5PSI. Then 10PSI, but give it a few minutes between inflation to see if the pressure drops or you can identify the leaking area. A fast way other than soapy water is also to just submerge in a tub of water. You’ll see bubble fairly quick in that instance.
It’s possible you have some residual grime/oil/metal shavings/debris that are plugging it up temporarily, too.