One setting to check @Claire246 is your booking window. If you have a five month booking window, your second dates are outside your booking window while the first dates are inside your booking window. Booking window is found under Availability Setting under preferences. See if that is the issue.
BTW, you do not have a security deposit and I would suggest you consider having a substantial one for an entire house. Even the best guests can have accidents that cause damage and it is easier to get money through the security deposit. Since you are new, you might not have read the Help Center about security deposits and claims. Just a thought