You're not going to want to hear it ... but wear a proper cover garment.
I've met and listened to all manner of folks who like to think that they
can effectively conceal most handguns under a T-shirt. However, standing straight and carefully posed in front of a mirror doesn't really simulate most normal activities, postures and motions we engage in during our normal lives.
Even a button-type shirt, worn untucked, can often be worn in such a way (size, cut and nature of material used) that it fails to effectively conceal a holstered pistol under many conditions, activities and bodily movements.
Some folks might have more luck than others, depending on how they prefer their clothing to fit, the styles/cuts of clothing they choose, their daily activities and how well their chosen carry methods work with their body shape/composition.
I remember the first time I saw one of our guys wearing a G26 in a IWB/tuckable type holster, underneath a tucked T-shirt. He was standing in line at a credit union. I was standing a few feet behind him, off to one side, and I could clearly the outline of his concealed Glock subcompact, and could tell he was using a magazine with a curved extension. The texture of the grip frame was even discernible underneath the shirt material.
Over the years I've usually found that it's not all that difficult to spot plainclothes cops (wearing untucked & loose shirts) who are armed with a belt-holstered weapon at training classes, conferences, seminars & meetings. Some apparently aren't concerned whether they're effectively concealing their weapons (especially if they're among mostly other cops), and some actually seem to think their weapons are really effectively concealed.
FWIW, lawfully concealing a handgun usually involves some adjustment for most of us, to some extent, when it comes to everyday actions, activities, motions, postures, etc.
Leaning, bending, reaching, twisting, kneeling and other motions may require a minor adjustment in order to continue to conceal a belt-holstered weapon. Think of it as a lifestyle adjustment, of sorts.
BTW, have you at least tried reaching with the arm & hand on the non-gun side?
Simple things ...