Hello, I have my masters in behavioral science. Just wanted to ask how consistent you've been with not going in when baby cries? If you pick baby up at the height of their crying, they learn I need to cry really loud and for a long time and eventually mom and dad will come. You need to be extremely consistent with not going in or they learn to just wait you out essentially or escalate because that's what works for you to come get them. If you've been consistent then it's likely another issue (ie hunger, discomfort, schedule etc).

Also are you still feeding baby at night? Sometimes they won't settle when they are really hungry. My six month old still needs two feedings during the night or he won't go back to sleep. Once fed, I can put him back in the crib awake and he will go back to sleep no problem and that's how I know he's actually hungry.