This is true for both large pharmas and small biotechs. Every contract manufacturer has a finite capacity, whether it is defined by equipment, labor or other factors. A reputable company should be upfront in telling you whether they can accommodate your production needs, schedules and requirements. They should also be able to explain how they can do it. Expansion plans can be touted, but make sure that they are being carried out, and bear in mind that it can take 12 to 24 months to get a line up and running.
Regulatory issues can hang up even the best-laid plans.
Be sure to review recent regulatory inspection records for observations of non-compliance. Quality is another key component in reliability of supply. The company's quality control program should also support its customers' efforts in obtaining approvals from the FDA and worldwide regulatory agencies, and their project management system should be geared to putting quality first. When you are interviewing candidates, find out from their operational teams how they respond, not just to a perfect situation, but to problems and challenges that may come up. This is what makes or breaks a schedule.