ZOA CTLR DESCRIBED CONFLICT AT 10000 FT WHEN PLT FAILED TO ACKNOWLEDGE ALT CHANGE AND FREQ ASSIGNMENT BECAUSE OF WX FACTORS AND ICING.

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ACFT X WAS ON IFR CLRNC SEBOUND ON V107 AT 9000 FT. DUE TO XING TFC ALSO AT 9000 FT; I ISSUED CLB CLRNC TO ACFT X TO 11000 FT. THE PLT ACKNOWLEDGED AND I OBSERVED A CLB. THE PLT REQUESTED APPROVAL FOR DEVS AROUND WX HIS 'STORMSCOPE' DISPLAYED; AND I APPROVED THE DEVS. HE ASKED IF I STILL NEEDED HIM TO CLB; AND I CONFIRMED THAT HE WAS TO CLB TO 11000 FT. WHEN THE MODE C READOUT SHOWED THAT ACFT X WAS ABOVE 10000 FT; I INSTRUCTED THE PLT TO CONTACT THE NEXT CTLR; THE PLT ACKNOWLEDGED. 2 MINS LATER I OBSERVED THE MODE C READOUT OF ACFT X TO INDICATE A DSCNT; CONFLICTING WITH ACFT Y WHICH WAS STILL AT 9000 FT. I CALLED ON MY FREQ TO SEE IF THE PLT WAS STILL WITH ME; BUT GOT NO RESPONSE. I CONTACTED THE NEXT CTLR; AND WAS TOLD THAT ACFT X NEVER CHKED ON HIS FREQ. I CALLED ACFT X AGAIN; AND THIS TIME HE RESPONDED. I INSTRUCTED HIM TO DSND TO 8000 FT; AS HE WAS NOW BELOW ACFT Y; THE CONFLICTING 9000 FT TFC. I REPEATEDLY TRIED TO COMMUNICATE WITH THE PLT; BUT HE WAS RESPONDING INFREQUENTLY; AND WITH LIMITED ANSWERS TO MY INQUIRIES ABOUT CURRENT ALT; ETC. WHEN I ASKED HIM IF HE HAD TRIED TO CONTACT THE NEXT CTLR; AS INSTRUCTED; HE SAID HE NEVER HEARD THOSE INSTRUCTIONS. IN THE NEXT 3 MINS I CALLED HIM REPEATEDLY; ONLY TO FIND THAT HE HAD CHANGED TO THE NEXT CTLR'S FREQ. ABOUT 2 HRS LATER I WAS ADVISED THAT THE PLT CALLED ON THE TELEPHONE AND SPOKE WITH THE WATCH DESK SUPVR. THE PLT SAID THAT HE WAS IN A STRONG DOWNDRAFT; HAD AIRFRAME ICING AND THE RADIO ANTENNA MUST HAVE ICED UP AS WELL; SO HE COULD NOT COMMUNICATE WITH ME. THE MAIN PROB WAS THAT I WAS NOT GETTING ANY VALUABLE COM FROM THE PLT AS HE DSNDED. HE ACKNOWLEDGED A FREQ CHANGE; BUT APPARENTLY DID NOT SWITCH OVER. WHEN ENCOUNTERING ICING/DOWNDRAFT PROBS; HE DID NOT ADVISE ME (OR THE OTHER CTLR) WHAT WAS HAPPENING. IT WAS ONLY OUR GOOD FORTUNE THAT HE WAS NOT DIRECTLY OVER THE OTHER ACFT WHEN DSNDING. IF AN ACFT IS EQUIPPED WITH WX RADAR; AND PLTS WISH TO DEVIATE AROUND WX DEPICTED BY THEIR RADAR; CTLRS WILL ALWAYS ALLOW THE DEVS; USING ALT TO ENSURE SEPARATION. IF A PLT DETERMINES THAT THE ASSIGNED ALT WILL PLACE THE ACFT IN CONTINUED WX ENVIRONMENT; CTLRS WILL WORK WITH PLTS TO DETERMINE AN ALTERNATIVE COURSE OF ACTION TO AVOID ICING; TSTMS; ETC. THE PLT MUST; HOWEVER; COMMUNICATE WITH US TO ALLOW US TO HELP THEM!