USPS taking forever to deliver certain invitations! Could they be lost?

Hello! Wondering if anyone out there is having issues with the USPS. I mailed my invitations in December (during the holiday rush which wasn't the best idea but my wedding is in March so it kind of had to be sent in December.) We have received RSVPs back from many folks from states across the country but my sister and a bunch of others I'm finding out STILL have not received my invitation! I'm panicking naturally since my invitations were so thoughtfully chosen. We had them weighed and got the proper postage for each one and they were hand stamped at my local post office. The post office says there is no reason they should be lost ? I am reading online that Calligraphy could be the problem. they are 100% legible but perhaps the machine can't read the state bc I had her write out the name of the state instead of using the 2 letter abbreviation. No invites have been returned to us and the return address is clearly typed out on the back. Any advice? I am baffled. Does mail really get lost