Sliding bars into single bolt quill stems is a matter of finesse, rather than force.

take a look at the stem and you'll notice that part of is sculpted away, usually near the bottom or bolt. that provides relief to clear the inside of the curves. It's like the difference between driving a truck around a corner in NYC where everybody parks right to the corner, and other places where they park back from the corners a car length or so.

As you thread the bar rotate the stem to keep the narrow section on the insides of all the curves. Sometimes the end of the quill will touch the bar blocking you, so rotate it back 270 degrees to get the same result.

If the bar has a sleeve extending on either side of the center, you can spread the stem slightly. If one side is threaded, remove the bolt and put in in from the other direction with a penny across the slot, and jack the stem open, but only enough to not scratch the sleeve. If the stem uses a bolt and nut, use a cone wrench to gently flex it open as you slide it up the bar.