First off, congrats on the new baby chicks!
-So you did good on lockdown which was on 2/8 at 7pm. Day 21 would have been 2/11 at 7pm.
-Humidity depends on location, but if I had to take a guess I'd say that you kept humidity good.
-I'm assuming your little giant incubator doesn't have a fan? if it doesn't then it's considered a still air incubator. The thing with still air incubators is that the temperature needs to be higher than the ones with fans (forced air). These still air incubators like the little giant without a fan require temperature of 100.5f to 101.0f at the top of the eggs during incubation. If I'm assuming correctly that yours didn't have a fan, then that could have been a reason these chicks were not ready to hatch at day 21 (2/11). The temp of 99.5 is considered low and would delay the hatching of the chicks for a couple days or so.
-Tips that I recommend is that before you try assisting, make sure you candle before you do. I would only consider to assist if the chick had internally pipped but has not externally pipped for more than 24hours. I would also consider assisting when the chick has pipped at the wrong end, but I would not take a huge chunk of shell. I would simply make the hole a little bigger for them get more air and then let them do its thing. Another reason I would consider to assist is when the chick has externally pipped and has not started to "zip" after 24 hours especially if you suspect shrink wrapping. Other than that, there is no other reason to assist at all. If you candle and you see the chick moving slightly (since there is not much space, but you should still be able to see a slight movement) after day 21 and you don't see any pips, that doesn't mean you need to assist. It means the chick is alive and is not ready to hatch yet, if you start removing shell you will still see those blood vessels which then puts the chick at risk for death due to infection or bleeding out.
I am starting to be more active on these forums so whenever you head to your next incubation and need some help or guidance, you can message me or mention me by putting
@kyeschicks on any post and I will help you! I'm sorry you had to go through this on your incubation, but I hope it helps that nobody is perfect and most people who try to hatch for the first few times (including myself) have made mistakes. I have assisted too soon when I first started hatching chicks and because of that I did lose chicks. My last batch of chicks that I hatched a few years ago, I did have to assist and I successfully assisted in a couple chicks that needed help by following my advice I gave you on this post. I'm currently writing an article that will help those on how to incubate eggs properly and improve hatch rates

Again, congrats on your baby chicks and please post pics!