I can get texts, but the other person isn't getting mine?

I have a Samsung Galaxy S5, the person who I mainly text (iPhone 4) isn't getting my texts, but I can get theirs.

i can text anyone else and it works though. It's been happening for about 3 days now. It says that they re being sent, but the other person isn't getting them.

Fgeted

If the other person is using an iphone, have them go into settings and make sure Send As SMS, MMS is turned on and Imessage is on. Then they should receive your texts with no problem