Why are some people not receiving my text messages?

[Android Central Question]

I have a Huawei P Smart and I'm wondering why some people can't receive my texts. My mom has a really old Samsung "keyboard phone" (not sure if that's a correct term, but it's not a smart phone) and she hasn't been able to receive my texts for a few weeks now. There's also another person who doesn't receive my texts, but I don't know what type of a phone he has, because I don't know him personally.
Since there are two persons who can't receive my texts, it makes me think that the problem is with my phone. However, some people, like my father, do receive my texts. He has a Samsung Galaxy A3.
So what could be the problem? I even went and asked at my local phone store, but they weren't interested and said they didn't know. I tried to research this online and did several things based on the info, but none of the things worked.
I have already removed and inserted my SIM card again, checked that I haven't blocked anyone (my mom hasn't either) and checked the signal status in my area. This problem has been going on for at least two weeks now, but before that everyone received my texts. Hopefully someone could help. Thank you

 

[Rukbat]

First, it has nothing to do with your SIM card (if the card were bad, you wouldn't have service at all), whether you've blocked anyone (that's for incoming, not outgoing) or the signal status (same as the SIM card).

Go into your texting app and look at its settings. If you find something like "don't break long texts" or "send long texts as MMS", turn that off. Some of the older phones did SMS, but not MMS. (Well, actually we've had some form of MMS since about 2002, but the multi-mapping wasn't fully operational until years after that, and an older phone [or a newer one built on the older standard] may not be able to handle your MMS messages - and that's how the phone keeps from breaking long (longer than 140 octets) SMS messages.)

(The setting may say "send long messages as individual messages - if it does, enable that.)