K-pacs server fails to receive images

We have set up a k-pacs server to receive the dicom images from a few hospitals.
The K-pacs server works in the past. However, in recent weeks, we have this problem.
Hospital has successfully sent out the images, but our K-pacs server didn't receive them and
it seems these images are being held somewhere. After we stop, then restart the server again,
we receive these images. Does anyone know this issue? thanks.