Turkish President Recep Tayyip Erdogan did not get a meeting with US President Joe Biden at today’s NATO summit in Brussels, despite Ankara’s rising stock amid the Ukraine crisis.

Biden’s packed schedule was cited as the main reason. However, Biden held bilateral talks with French President Emmanuel Macron. Turkish officials and the US Embassy in Ankara pushed hard for a meeting, if only a pull-aside on the sidelines of the summit, according to diplomatic sources speaking not for attribution.

The real reason is Turkey’s refusal to get rid of its Russian S-400 missiles, which the United States says threaten NATO security. Turkey acquired the system despite unremitting US pressure to scrap the deal. A slew of Congressional sanctions and its ejection from the F-35 joint strike fighter program have not swayed Ankara.

The Ukraine crisis was seen as a possible off ramp, with US officials reportedly floating the idea for Turkey to relocate the batteries to Ukraine. Turkey remains adamant that it will retain the S-400s, a message made clear in an op-ed penned by Erdogan’s communications director Fahrettin Altun that ran in the Wall Street Journal Wednesday. Altun called the transfer of the S-400s to Ukraine in exchange for easing of sanctions an “unrealistic idea.”