Dear Damindra,
a "D" means that your computer was so busy, the operating
system wasn't able to handle network data sufficiently fast, and
in effect, network packets were dropped even before they reached
UHD.
This means that your filter is too compute-intense for your PC.
Either get a faster PC, or decrease the order of your filter.
Overly sharp filters are a common mistake.
Best regards,
Marcus
Also, for regularly-spaced channels, using a PFB channelizer or PFB
synthesis block may be significantly more efficient for what
Damindra is