Should I be worried if my letters havent been delivered?

I sent a letter, and pictures to my bf that's in county jail in ullin il, I sent it on Monday and for some reason he still hasn't received it, but all other mail that I've ever sent him he gets it in about 1-3 days, but this time he hasn't got it. I've checked my mail to make sure it didn't get returned or anything. So, do you know what the issue could be with him not getting it yet? What could I do to figure out what happened? It has lots of pictures of our kids in the letter I sent, so I'm just trying to figure out what happened because any other time he receives the letters within a day or two.