[General] problem with proteus running a fulladder

[ellen]

Hi

i designed a fulladder in proteus.it is working.
but when i gather them in one block.one of the logic probes that present the outputs digits show a qoestion mark(high impedans)i think
why??
confused by this happening.

 

[Vijay Karthik Kumar .A]

Could you post the schematic for better reference?