Rotor bolt heads are shallow. There’s not much insertion depth for those little T25 wrenches. In fact, that’s why they use T25 in the first place. They offer more surface area than would an equally shallow 4-millimeter hex, so theoretically there’s less chance that they’ll strip out. But being a rotor bolt means exposure to mud, water, heat and torque. Sometimes, you’ll find one that needs more force than that little six-pointed star can offer.
Hopefully, it’s just one. A last-man-standing between you and your rotor. If that’s the case, we’ve got a way to mend it. Start by removing the five bolts you’re able to. To get that last one, you’ll be rotating the rotor itself to break the seal. You’ll need to turn the bolt at the same time as you’re turning the rotor, so have the wrench inserted and ready. The rotor may need to be lifted if there’s any kind of cosmetic raised shoulder inside the hub’s six-bolt diameter. If the hub’s end cap (that’s the little cylinder that spaces the hub from the frame) is removable without taking off the rotor, you might as well pop that off too. It’s probably not necessary, but it would allow you to turn the rotor 360 degrees, so you might as well do it now.
The key here is synchronization. Turn the rotor with one hand and the wrench with the other, each with the same force and speed. You may have to rock both back and forth a bit, but you should eventually be able to get the bolt to loosen independently of of the rotor and come all the way out. When it’s time to reassemble, clean the hub’s rotor interface well and use six fresh rotor bolts, ideally with a fresh coat of blue thread-lock.