Has anyone had issues with their 3PL?

I operate an NC based 3PL that specializes in kitting (doing handwork, assembling multiple components from different suppliers into a single package with custom branding/packaging/inserts etc. and sending it to the end user) warehousing, and fulfillment. I have a great book of steady business but I want to start ramping up and scaling this out. I am struggling to find more businesses who need this niche service because most people either already have contracts with larger companies or are keeping everything vertically integrated.

This brings me to my question: How did your 3PL cause problems for you? Did you solve them or did you just bring your fulfillment back in house? Help me learn from their mistakes so I can create the best service offering possible for dropshippers and scale my business up.