As above, the absence of a pin shouldn't cause it not to latch.
If it's working without the latch I wouldn't worry too much.
You can try pushing it reasonably firmly (think of how hard you push RAM or a graphics card in, then apply about 2/3 of that pressure), but obviously don't force it too hard. It will require a fair bit more force than the other connectors.
If you're worried you'll cause damage by pushing too hard, you can instead try 'see-sawing' it it by pressing harder on one end and then the other sequentially until it's all the way in. Takes a while (because you don't want to angle the connector too much in one go), but it puts less pressure on the connector and the motherboard.
And, because I've not explained that very well, I'm gonna treat you to some of my leet Photoshop skillz: