Cistern always empty despite rain

Hello,
I hope I am asking in the right place.
We have a problem in our new building that our 7000L cistern is always empty.
Despite 2 days of rain, there is so little water in it that the pump does not even deliver it upwards.
The rain from the gable roof with 200m2 should actually make a difference.
The neighbors with cisterns all have water.
Where could the water be lost?
The downpipes would overflow if they were clogged.
It is a concrete cistern that was installed in one piece.
What comes to your mind about this?
Where could the mistake be?

You don't have that many options. Check the [Zulauf].

Was it tested whether the cistern is watertight?

Either nothing flows in, or what flows in flows out. Pour the watering can into the gutter and catch it again in the cistern to compare.

I would also rather say that your concrete cistern has a defect somewhere. If it rains for two days, there should already be enough water in it for the pump to work. At least with a 200 sqm roof area. But as already described, pour water in at the top with a watering can, and check at the bottom if about the same amount comes out.

We will do that right away today. I don’t know if the tightness was checked. First of all, thank you very much for the help.