I send texts but they sometimes don't make it to the sender?

I have an iPhone 4s on current ios.
Having problems sending text messages, not iMessages
Text sends and doesn't ever show "failed" or "not delivered"
Receiver doesn't get them all the time.
The receiver has an android.
Is there a fix for this? Why aren't they being delivered when my phone shows it sent successfully?

Text messages aren't as reliable as iMessages. There's no way to tell if your text message will actually reach the sender because many different issues could occur from carrier to carrier. Your iPhone is not the problem, and the receiver's phone may not be the problem either. The problem would exist with the network. Text messages happen to get lost frequently.

One situation was of mine and my friend, our conversations consisted of multiple text messages back and forth and I hardly ever got all of the messages. This eventually turned into us both getting iPhones so we would use iMessage and no longer have the problem that text messages created.

The issue of your text messages not being delivered lies with the carriers.