However, when a player enters the same funnel and is pushed against the button, it does not depress.

How do I fix this?

It isn't anything fancy. It's just an excursion funnel that ends dead center on a prop_floor_button. Like I said, cubes activate it, but a player does not.

I've never heard about a button not able to be deactivated but if the player can't activate it at all, try removing the funnel and walking on it to see if it does.

If the Excursion Funnel is there because you 'portalled' it from another wall, try putting info_placement_helper's to set the portal to the middle of the wall right above the button and one in front of the Funnel.

If nothing works then decompile a map which a mapper has already done this technique and see if anything is different to what you have it.