Let them go the distance of day 25. Very little hope after that though it's been known to happen. It's not much of a worry that late in hatch for low heat few hours. I believe your trouble is low heat the entire hatch. For example my thermometer is all calibrated and always have hatching right around day 21, either starting day 20 or day 21. That said we've had to do staggered hatches all spring meaning I've eggs in turner and eggs laying flat where some turner rails are taken out. That lower area even though a fan unit is cooler. In the course of 6 days that near or one degree cooler sets them back to hatch day 22 and 23.
Two things could have caused your lower temp. First and often the case is your using a still air incubator. Those have serious layering of temp and is recommended to measure top level of eggs 101.5 F if upright like in a turner, 100.5 F if laying on side like your manually turning. As you see that's quite a bit different than 99,5 F and will delay a hatch dramatically or ruin it if you've been driving for that lower temp. The other cause if that's not it is simply your thermometer is off. Use an oral thermometer to calibrate yours or run a full degree higher next hatch. If on that hatch they don't zip and hatch until day 22 or 23 then incubate 1.5 F higher than what you did this hatch.
Inaccurate equipment is fine to use as I've seen nothing spot on excepting an oral thermometer or $100 lab thermometer. Adjust temp accordingly each hatch until you know what it should read for them to start piping day 20 and all of them hatched day 22. Then you know it's right and can move on to other things that lower hatch rate. Temperature is the most important thing for successful hatching. Few hours is nothing to worry about, a degree or more low for 21 days is another story.