Viacom CEO Philippe Dauman will be playing with fire if he tries to sidestep questions about Nickelodeon’s declining ratings on Monday when he addresses the annual UBS Global Media & Communications Conference: Even some of his company’s more faithful supporters are beginning to wonder what the hell is going on. Today, Nomura Equity Research’s Michael Nathanson joined the pack, decreasing his price target for Viacom shares by $1 to $54 based largely on what he calls the “inexplicable declines” at the channel. He says that Viacom’s total ad growth could be as low as 3% in the last three months of 2011. The reason: Nick’s contribution to Viacom’s ad sales jumps from about 15% most of the year to 25% during the holiday season as companies turn to the kids channel to market toys, movies, and gifts. Nathanson says that’s a problem now given that Nick’s ratings “are down in the high teens, and that the remaining networks are also down (by about -4%).” The poor results plus Viacom’s refusal to provide guidance on ad expectations “remains the core controversy,” Nathanson says. (He still has a “buy” recommendation for Viacom, in part because its plan to repurchase $10B of its shares offsets the new earnings risks.) Early this month, Dauman described the drop in Nick’s ratings beginning in September as a “blip” largely attributable to problems with Nielsen’s measurements — a charge that the ratings company rejects.