At the Enhancement Forge building you have improve your weapons to make the stronger and more effective.
To enhance weapons, select a hunter, and then select the weapon that he owns that you would like to enhance.
When you hunter arrives at the Enhancement Forge, tap the speech bubble and on the next screen your'll see all the stats in relation to the weapon enhancement. In particular, the chance of success!
Enhancement Forge - Improve your Weapons and Armor
When you make the first few enhancements the chances are pretty much 100% success, but as your materials decrease and the enhancement goes to higher levels there is some chance of failure.
You have the option to try multiple enhancements to save time, and you just need to stop them if you notice that you are starting to get fails.