It has to be a lance, and you have to be going at a certain speed. In the lower right there is a dot next to your weapon icons. I think it is usually red, and it turns grey when you are going fast enough to couch. It's either that, or it's grey and it turns red when you can couch. Lol.
Press X when you reach a certain speed, like Squints753 said, when you reach a speed within the range the little red dot in the bottom of your screen turns grey, be warned though it no longer stays couched indefinitely, it only lasts a few seconds and you have to wait for it to "recharge" same applies to slowing down, it raises the lance
If that still doesn't work you are probably using the wrong type of polearm, make sure it makes mention of it being a lance in the description