A/F sensor replacement troubles

So, I went to replace my air fuel ratio sensor yesterday. I got the old sensor off and disconnected, and the new sensor (Bosch 15974) screwed in, however, I can not for the life of me get it connected. It goes in about half way and stops. The connectors look nearly identical. The original sensor connector fits on almost effortlessly. What gives? Has anyone else had issues with this? The only three things I can think of are that the pins aren't perfectly aligned, that a plastic piece isn't just the right shape, or that the connectors have changed shape or size due to age. Any recommendations?

UPDATE: Disassembled both connectors and compared them. There's very small differences in measurements, according to my caliper. I replaced the connector on the new sensor with the one from the old. It snapped in effortlessly. I'll update later.