Hi Dennis and welcome. Failure to fire in a Browning Buck Mark Plus UDX is most commonly caused by a rounded or broken firing pin, a dirty firing pin channel, or degraded/weak springs. Immediate steps include thoroughly cleaning the breech face and firing pin channel, checking for a disintegrated recoil buffer, and testing with different, high-quality ammunition.
Firing Pin Issues: The firing pin on the Buck Mark can become "mushroomed" or rounded, resulting in light strikes. Replacing it with a hardened steel firing pin is a common fix.
Dirty Firing Pin Channel: A buildup of carbon or unburned powder (crud) inside the action and around the firing pin channel can prevent full penetration.
Broken/Degraded Buffer: The recoil buffer can break down over time, particularly in older models or with extensive dry-firing, and the pieces can obstruct the hammer.
Sear Spring Misalignment: If the long leg of the sear spring is not properly centered in its "V" groove, it can cause ignition failures.
Aftermarket Parts: Over-tightening sight rail screws can bind the slide, preventing it from fully closing.
Ammo Sensitivity: Rimfire ammunition can sometimes cause light strikes due to inconsistent priming or harder metal, leading to inconsistent ignition.
Recommended Steps:
Deep Clean: Clean the breech face, firing pin channel, and under the extractor.
Inspect Firing Pin: Check the tip of the firing pin for damage.
Check Buffer: Ensure the recoil buffer is intact. Cheers!

A sudden failure to fire in a 10-year-old Browning Buck Mark Plus is likely due to a dirty breech face, worn-out main spring, or a "mushroomed" firing pin. Trying different ammo was a good first troubleshooting step, but since the problem persists, mechanical or maintenance issues are more probable.
Here are the recommended steps to diagnose and potentially fix the issue:
1. Perform a Thorough Cleaning
Accumulated carbon, lead, and gunpowder residue on the breech face (where the back of the cartridge sits) is a very common cause of misfires in these pistols. This gunk can prevent the cartridge from seating fully, cushioning the firing pin's blow and resulting in a light strike.