the most difficult leak to find...is inside the regulator. If you put a little bit of soap solution on the weep hole of each of the regulators...does that one bubble more than the others?
Inside the regulator there is some type of vavle that allows pressure to go from the High side to the low side. If this leaks...you'll vent gas out the weep hole. It also can cause the regulator setting to creep from it's original set point.
If your regulator uses a "capsule" vavle seat...it could be loose or faulty...
http://www.micromatic.com/draft-keg-beer/regulators-parts-pid-8402.html
If your regulator uses a poppet valve (imi cornelius) it could be a bad poppet or the mating surface. The mating surface is pressed into the brass body...it has a thin o-ring that makes a seal. Sometime this seal leaks... With an IMI Cornelius regulator...if you unscrew the set screw to drop the pressure to zero...and take the front cover off (4 -screws)...you'll see the poppet in the center of the regulator body. With the tank valve ON...there should be NO gas coming through the poppet with the face of the regulator removed. Test it with soap solution. If the poppet leaks...you can get a new one in a rebuild kit. If the seat mount leaks (were the silver colored valve seat meets the machined brass regulator body...you're probably out of luck. You may be able to press it out carefully, apply some NSF silicone sealant to it, and press it back into place. If it deforms at all...it'll never work again)
Note: if you have an IMI cornelius regulator...there IS a difference in the rebuild kit for a Primary regulator and a secondary regulator...make sure you get the correct one.