The most common cause is cloud. ORBIS will not deliver an optical product where cloud cover in the acquisition exceeds your configured threshold, and it will not interpolate through cloud and present the result as observation.
Check the task status in ORBIS Viewer or through GET /v3/tasks/{id}. A task in state AWAITING_WINDOW is waiting for a feasible acquisition opportunity; ATTEMPTED_CLOUD means an acquisition was made but rejected at quality control.
If you need a result regardless of cloud, consider a SAR source. SAR sees through cloud and answers a different question — it is not a drop-in substitute for optical.