There are a number of reasons why this is the case. I probably can’t list every reason, but will do my best to answer the most likely reasons. In all instances, if you are scams, or SPAM – I will not reply. If you are in a hostile or vulgar manner, I will not reply. If you do not leave a means to reach you, I can not reply.
For email, if you only write one or two sentences, I will not reply. If you are not going to put effort in writing, I am not going to entertain you by replying. It will be good to at least write one paragraph. If you are writing an email for the first time, and need recommendations on writing a good intro, consider sending an email to email@example.com. You can send a blank email, and this is an unmonitored auto-reply.
For Messaging, or SMS, if all you are going to do is write OK or Yeah, then I will assume you lost interest in the messaging, and will lose interest in trying to keep the conversation going. If you are not in my contacts, and you send an SMS to my private cellular number, I will not reply. You can send an SMS or iMessage to +1 412 240 9901. This telephone number is not a voice line. For that, you would need to call.
As for telephone calls, if you are not in my contacts, and call my private telephone number, you would go straight to voicemail. If my phone is in DND mode, then you will go to voicemail. If you do not leave a valid message where you give your name, a telephone number, and a reason of why you called, I will not return your call. If your telephone number is not in my contacts, you must call me at +1 800 921 7610.