FLC OF AN MIL WDB FAILED TO FOLLOW THE LDA LOC COURSE DURING APCH RESULTING IN TWR INTERVENTION TO DIRECT THEM BACK ON COURSE AND PREVENT OVER RUNNING THE OTHER PARALLEL RWY FINAL.

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I WAS ACFT COMMANDER SITTING IN THE R SEAT (NOT FLYING) ON MIL. WE SET UP FOR; AND WERE GIVEN AN INTERCEPT HDG TO COMMENCE THE STL LDA/DME RWY 30L. AFTER INTERCEPT AND INITIAL TRACKING OF THE FINAL COURSE; THE PF (L SEAT) STATED THAT HIS CDI NEEDLE APPEARED TO BE FROZEN IN PLACE WITH NO WARNING FLAG ASSOCIATED. A QUICK COMPARISON BTWN THE L AND R INSTS VERIFIED THAT AND I TOLD HIM TO MAKE A R CORRECTION OFF MY CDI WHILE I CHKED THE PROB. SHORTLY AFTER; WE GOT A CALL TO COME L 15 DEGS BECAUSE WE WERE GOING R OF COURSE WHILE PARALLEL APCH WAS IN PROGRESS TO RWY 30R. WE IMMEDIATELY TURNED L 15 DEGS AND THEN BROKE OUT UNDER THE WX AND VISUALLY ACQUIRED RWY 30L AND CONTINUED VISUALLY. AT THE TIME OF THE RADIO CALL TO TURN L; THE R COCKPIT CDI WAS SHOWING ON COURSE AND I STATED THAT WE WERE SHOWING ON COURSE AND ASKED FOR VERIFICATION. WE WERE TOLD THAT WE WERE IN FACT ANGLING R OF COURSE. THE ENTIRE TIME (ALL THE WAY TO TOUCHDOWN) THE L CDI REMAINED FROZEN ON COURSE DURING ALL THE TURNS. I DO NOT BELIEVE THAT WE EVER ENCROACHED UPON THE R RWY APCH CORRIDOR. AFTER TROUBLESHOOTING THE ACFT AND DISCUSSION WITH MAINT WE BELIEVE THAT WE EXPERIENCED DIFFICULTY WITH THE 110.55 FREQ. ALL OTHER FREQS USED THAT DAY ON 3 OTHER FLT LEGS WORKED FINE.