Pitchers really try to nibble when it comes to star players in this game (20 at least...I don't have 21).
I'd agree with the above posters re: sample sizes, pressing, etc.
My edited Vlad Jr. has the best offensive ratings on my franchise. When I'm hitting with him, and there's nobody on, pitchers are throwing all kinds of junk, trying to get me to inevitably swing at a pitch that is going to end up with less than ideal contact, or for them to just walk him. It's pretty unusual to see him get something middle-middle, and I definitely struggle to mash with him.
If I can get someone on base in front of him though, pitchers seem to be far more agressive attacking the zone, and have more of a tendancy to hang one, resulting in his power numbers coming through.
I don't know if it's been mentioned or not yet, but batting stance sometimes has a lot to do with it as well. For instance, I've used Naylor in the past (And Shed Long in Seattle), and now have Willie Calhoun in my lineup. None are rated super great, but I just find that they have all have stances that help me to see the ball/make perfect contact with. Others, like Bo Bichette for instance, are guys that I'll typically struggle with. Guy his contact ratings in the 80's, but I'm hitting .220 with him in mid-August.
Back to the sample size thing...the temperature also affects power numbers. I get that you've got a couple of guys mashing early, but with someone like Ramirez, if he's coming up just a bit short right now, he'll probably be mashing through the heat of the summer.