Talk to the players.
There are mechanics and practices you can use to soften this up, but randomization will inevitably lead to occasional failure. I've got some links at the bottom of this post that you might be able to use in specific situations, but ultimately this is just a hazard of the dice-based RPG experience. It's not an experience to be trivialized though, so I've got a lot more to say:
Fudge it.
I once let players drastically decrease an enemy's armor by taunting her into a rage (despite having no game mechanic to justify it), just because the fight was going to be insanely harder than I intended otherwise.
Just fiddle with the numbers. Grant bonuses and penalties on the fly, etc. There's a link or two at the bottom about how to do this gracefully.
Fudging isn't useful in the long run, but if you just run into the problem once or twice per campaign then it's acceptable.
If you want a gentler game, change systems or institute houserules.
It's more or less possible based on the system, but in D&D 4e we were able to design PCs who never roll attacks (a warlord/shaman hybrid who granted attacks to others, and a magic missile wizard), and one who exploited Hammer Rhythm to never care if he hit or not.
Other people have similar problems.
Some of these questions aren't exactly your issue, but the answers might help!