How do I ensure that the Zentrum joins the DU coalition?

This is my second playthrough of the German Empire.

I planned everything in considerable detail, watched a few guides, and feel well-prepared for the German Empire. Everything went according to plan; I managed the card game reasonably well, the Ruhr conflict went as planned, but the SPD ousted Schleicher too early, and the Center Party wasn't in the coalition, even though I had already elected the theologian Carl Ulitzka as chairman and the SPD had always targeted the Zentrum (left).

What did I do wrong? I would be incredibly grateful for any tips! (The coalition also included the Minority Bloc and the LVP.)