Depending on how you attach your warp to your back beam/ back warp-stick on a rigid heddle loom, you can find that when you’re close to the end of your warp the warp isn’t all nice and level. Some warp ends are higher/lower than others and this creates a messy gap which can make it hard to get a clean shed.
My approach when this starts to happen is to get a nice long bit of scrap yarn and, just in front of the back warp stick, wrap it around bunches of warp ends. This cinches them together and removes the gap. I find I can squeeze the very last out of the length of my warp this way.