I can relate, because that once cost me a trophy buck.
First I'm assuming that your primers are good and there was no firing pin strike strike after attempting to fire. If there was it could be bad primers. If that's not it, then look to one of these causes as the possible culprit:
1.The firing pin assembly is dirty or broken. Check to see if you have a strong firing pin hit by looking at the primers of cases that fired. If not,clean thoroughly, and lube with light oil. If that doesn't solve it see if it's broken.
2.TYhe action is locked up. Both Encores and Contenders have a feature that prevents firing if the action isn't locked up tight. I've had this happen when the case it to long, either at the neck or the shoulder, or the overall length of the cartridge (OAL) is to long preventing the action form closing completely. What happens then is that the action is closed but is not locked up. In this condition the firing pin blocker stays activated. So, the hammer doesn't hit the firing pin when it falls.
3.Less likely, the bolt (the spring-loaded gizmo attached to the back of the barrel)isn't de-activating the blocker even when the action is locked up.
Diagnostics:
If it's failing to fire with handloads but works with factory loads it's likely #2.In which case it's a case length, case sizing or OAL issue. (See Note below).
If it's happening with factory loads, its likely #3.Then, the first thing I'd do is a thorough cleaning--to include air under pressure-- of the frame,bolt and firing pin. Something as inconsequential as a small piece of stray brass in the blocker mechanism could be causing the problem.
You can also test the piece for #2 by using one of the rounds that did'nt go off. If the action opens without having to pull the release lever it wasn't locked up. Then, if the same cartridge goes off after you snap the action shut smartly, it's likely the case sizing or OAL is at issue. But if it's locking up but not firing on factory cartridges, or proper sizeed/OAL handloads, it's likely the bolt-blocker issue.

Note: Case fit is more critical for break actions because they lacks the caming power of a bolt gun to force the cartridge into the chamber. That's particularly true if you make your own brass or resize other brass to fit (e,g. 25/06 or 280 from 30/06. For several of my wildcats, I start by trimming to specs but initially sizing the shoulder location so the action WON'T lock up. Then, I size it to fit the chamber tightly by progressively turning the die in a bit at a time, and trying for fit, until the action starts to lock up consistantly. Then, I lock the die and size the rest of the cases. That way I know it not only fits tightly but is headspacing off the shoulder, the back of the action and the rim,if applicable. Doing so makes for greater accuracy and my brass lasts much longer by not streching too much at the case web when fired.
If you know the case size is right, check for OAL inserting the cartridge into chamber(preferably with barrel off) to see if the bullet is hitting the lands. If it won't go in flush with the back of the baarrel, the OAL is to long. (There are guages by Stony Point, Sinclare, etc that help in setting the OAL.
If the sizing or cleaning doesn't work, get it back to TC,it's likely a mechanical problem.

Try reading this for some additional info - good luck.

http://www.bellmtcs.com/FAQ/Experiment.htm

I have 2 encore frames and multiple barrels (centerfire, muzzle loader, and shotgun.

I have NEVER had a missfire, EVER.

I would send it back to T/C, there is a problem.

I have a similar problem with my 30-06 pistol barrel. I feel my problem is that T/C cut the chamber a bit too deep. New brass will sit approx .003 below the breech. I will get the occasional mis-fire (about 3 - 4 out of 100). My frame gap is .004, so after I fire-form, I size the brass to protrude .003 from the breech. This procedure has elimated my mis-fire problems. I hope this helps.

I just experienced that same problem yesterday for the first time with my Encore. I was using a fresh bag of 25-06 brass that I trimmed to length and did up the case mouths.

I had three misfires out of 20 rounds yesterday. All had light primer strike marks and would not fire on subsequent tries.

I know how to size down shoulders, but how do you get them longer?

Joe