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Make sure that you have selected "Send as SMS". You can find that option under "Settings, Messages". It's toward the end of the options so you have to scroll down a bit to see it.
 
Are you doing this when you have a cellular connection?
 
Probably not your fault if you're sending it as SMS, and it really doesn't matter whether you're on WiFi or cellular. I have the same problem with certain folks that I message who have Androids.

I have always found that messaging an Android is hit or miss. I usually wind up having to email them if I have their address.
 
This issue has been around for years but only seems to affect a small percentage of iPhone users.

Have you checked Settings>General>About to see if there's a carrier update? If nothing shows up when you do then there isn't one.

As a test, if you turn off iMessage at Settings>Messages does texting to an Android recipient work?

Another thing to try would be logging out of iMessage at Settings>Messages>Send & Receive>Apple ID and testing texts to Androids. After that, restart the iPhone and then go back and sign back in at the same place. Then test texting to Androids.

A further step that you can try is Settings>General>Reset>Reset Network Settings but be aware that you'll probably have to re-enter WiFi passwords and re-connect to Bluetooth devices as the iPhone will forget them…and perhaps any VPN you've configured on the iPhone.
 
What I have found out is the Android user has gone from Android to IPhone and then back to Android it really stops some text messages. It is really bad if the text is part of a group text. It is an issue that Apple is never going to fix, as it is another reason many people “have to” stay on the Apple train. It would be great if Apple made sure your text was delivered no mater what platform you or the recipient were on.
 
What I have found out is the Android user has gone from Android to IPhone and then back to Android it really stops some text messages. It is really bad if the text is part of a group text. It is an issue that Apple is never going to fix, as it is another reason many people “have to” stay on the Apple train. It would be great if Apple made sure your text was delivered no mater what platform you or the recipient were on.
It's possible that iMessage might not be deregistered in some of those cases -- the users can double-check that via a site that Apple provides:
 
Are you comming from Android? I have heard people having problem going to iphone, if they didn't deactivate RCS (Rich Communication Service), which is a sms successor google is pushing, but Apple does not support.
 
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