I used to yank on the top of the seat and think it was tight then a tech showed me to test only at the belt path where the LATCH strap runs. Night and day. I click both connectors, then lift the cover or reach through the seat to pull the tail straight out from the adjuster while pushing down with my forearm. For forward facing I always add the top tether and snug it so the head does not pitch forward. If you are stuck after trying a few angles do not fight it forever, a seat belt install can be easier in some cars and is as safe when done right.
