URL Issues – some sites blank with www, some need www or they are blank.

First, please note I have ZERO programing skills. So all explanations should be written for someone who doesn’t know anything, ok? Thanks!

Ok, here’s the deal. I have over 14 websites with this same issue. I’ll use two as examples. theofficefurntituresupply.com and theskincareauthority.com. My problems with all my other websites are the same as these two.

What happens, is that if I type www. before one the domain name, in one instance it opens up the main website page and in the other it just opens a blank screen. The result is just the opposite if I don’t type the www before the domain name.

I need the page to open on both a straight https://domainname.com as well as a https://www.domainname.com. At this point, with no rhyme or reason, my sites do one or the other without any pattern to it.

I’ve talked with the hosting companies and they’ve worked hours on it. This is what they say.

“Upon examination of this issue, it appears that you account is using WordPress blogs which tend to URL rewrite based on entries within their respective databases. We would advised that you examine your WordPress configurations to resolve this issue.”

So, any ideas? I certainly need help! And please remember, I don’t understand programming language at all….so the help has gotta be written simply.

Thanks in advance for your help!