It was probably just a bad fit. That happens, a lot. This goes both ways also. A good independent developer with a lot of demand will say no to clients more often than saying yes. Knowing which clients to say yes | no to is a gut feeling which comes with experience. Sometimes you make mistakes and still go with a bad client, and that goes both ways also. Sometimes the person doing the hiring makes a mistake. We are all learning, don't take it personally, just learn and move on.

I don't know your situation and if you were hired as an actual employee or as a contractor. Either way, one way to check for good fit is to freelance with the company for a trial period. If it works out, go full employee. If not, keep looking.

If you were a little lost, then maybe you need to beef up your skills a bit. For example, if the project is a big platform_x code base and you know platform_x really well, then it's easier to jump in and be productive right away. Or maybe it's a platform you know but in a domain you don't know well.

There is a lot which goes into productivity aside from just code. Communication is important. Adapting to the existing work-flow is important. Make sure that you adapt to everything rather than resisting things you might not agree with or don't think is important. It's easy to slack on something you feel isn't important when in fact that thing might be really important for your manager. As you get more comfortable and start knocking stuff out, you can suggest changes.