Visa issue after submitting my COE

Hi everyone,
I've received my COE few days ago. I've applied for visa to the immigration today. They've received my passport and documents. But instead of giving me specific date they've told me wait for their call.
What would be the possible reason for this? Is there anyone who faced same issue?

Thanks in advance.

I've applied for visa to the immigration today

No, you didn't. Probably, you applid for a visa at a Japanese embassy, or maybe you applied for a change of status at an immigration office. Which is it?

Yeah I've applied to the Japanese embassy for obtaining my VISA. I thought they'll need 1 week time for stamping VISA to my passport. But it turns out that, it'll take longer. I was wondering what might be the reason of embassy to keep me waiting even though I've COE?