Struggling Pincushion Urchin

Hi, I bought a purple pincushion urchin on Sunday and introduced him via the drip method over a period of about 1hr 15m and when he went in the tank he was going round on all the rocks and the glass but gradually he’s been moving less and less and now his feelers are drooping and there’s some spines on the floor. He’s alive but I don’t understand what’s happened? Any ideas?

Ammonia: 0 nitrite:0 temperature: 25 degrees C