What can cause a Sylesti to fail multiple missions in a row?

pay close attention to the Recommended Stat

if your pet doesn't have a high number in the stat that the mission asks for, it's more likely it'll fail.

As Flute said, your pet may not be high enough in the necessary stat, whether through accidentally sending the wrong pet on the mission (have done this way more than I want to admit) or they are just not there yet. Although a certain level is required for the mission, having the pet at least a few levels above that is a nice buffer. This guide should help get your mission pets ready to kick butt.

Hope this helps