MD80 TCASII DID NOT SHOW ANOTHER ACFT.

Narrative

CTR HAS THE MODE C BUT TCASII DOES NOT SEE HIM. DFW DEP CLRED US TO 17000 FT AND TOLD US TO CONTACT ZFW ON 127.0. WHEN WE CHKED ON WITH ZFW THEY HAD US CLB AND MAINTAIN 16000 FT. TFC WAS A CESSNA 4114 AT 17000 FT. WE HAD THE TCASII IN THE ABOVE; 40 MI SETTING. NOTHING WAS SHOWING AT THE LOCATION GIVEN FOR THE C414 AND I ASKED THE ZFW IF THEY HAD A MODE C READOUT ON THE C414. THEY SAID THAT YES THEY DID. WE WERE VISUALLY SCANNING FOR THE TFC WHEN THE TCASII ALL OF A SUDDEN SPOTTED HIM AT THE 10 MI RING. THERE WAS NOT A LOT OF OTHER TFC ON THE SCREEN THAT THE TCASII WAS TRACKING. MAYBE 5 OR 6 AT MOST. ANOTHER COMPANY FLT WAS BEHIND US AND WAS HELD DOWN AT 16000 FT ALSO. HIS FLT NUMBER WAS Y AND HE ALSO RPTED THAT HE DID NOT HAVE HIM ON TCASII UNTIL HE BROKE THE 10 MI RING. THE CESSNA'S TAIL NUMBER WAS ACFT Z. I HAVE THIS HAPPEN SEVERAL TIMES A YR WHERE THE TCASII DOES NOT SPOT THE TFC UNTIL UNDER 10 MI AND SOMETIMES NEVER EVEN SEES THE TFC THAT THE GND CTLRS RPT HAVING A MODE C READOUT ON. MOST OF THE TIME IT IS ANOTHER AIRLINE THAT THE TCASII DOES NOT SEE. AWHILE AGO WE GOT A MESSAGE THAT BASICALLY SAID THAT THIS CANNOT HAPPEN; THAT THE TCASII SEES ALL AND KNOWS ALL. THE GND RADAR SEES THE TFC BUT THE TCASII DOES NOT. WHY IS THIS SO?