Trim the back edge of the barrel release where the extension hits it until it doesn't catch any more, the extension needs to be as close the tip of the spur as it can be to still allow the hammer to be cocked, there's only a little sweet spot where it will clear the release after you trim. If the hammer is the old style that doesn't use a transfer bar, the only extension that will work is the M94 type that requires the spur to be drilled and tapped as shown in the FAQs low profile hammer mods.
Tim