Samsung galaxy s2/s4 to Verizon issues. (Iphone maybe?)?

I had a Samsung Galaxy S2 and I've now upgraded to the S4. For the most part, my phone is fine, but I've been having trouble texting certain people. I believe they all have Verizon. They simply never receive my text, and I never receive theirs. However, certain times they will receive my texts, which makes it even stranger. For example, I may send out 1 text every day for 15 days and they will receive one. We chat for about 10 minutes, and then again, we never receive eachothers messages. I think they have Iphones, but I'm not sure. I can't exactly text them to ask. I've also tried using apps on my phone like text now, they don't seem to help. I have no idea if they have connection issues with other people as well. I don't think so though. I was wondering if anyone could help, or had any work-around ideas.