So on Wicker Man and other GCIs the issue is a combination of your height and larger thighs. On GCIs, if I have my feet flat to the floor, my thighs aren't flush to the seat and there is a gap between. Because my thighs are larger, the restraints won't lock. To get around it, I have to cross my legs, which puts my thighs closer to the bottom of my seat and the restraint locks no problem. It's why you'll often see people who are perhaps larger than me but shorter (I'm 5'11 so not some towering giant) get on with no issue, yet I have to do the leg cross thing. Interestingly Wicker Man was notoriously awkward to get on before it opened when I did the press preview event. Thankfully they appear to have made changes, as there were a lot of taller people who struggled then.
B&M hyper coasters quite frankly are some of the absolute worst to get on with larger thighs, even if you're not that "large" overall. I would get on Silver Star at my current weight, however it's incredibly uncomfortable because my legs have to be absolutely pinned in. Even when I was a few stone lighter years ago it was still a squash. It was the same with Steel Dragon 2000 and its B&M trains - I only did it once since it just wasn't enjoyable. Newer models such as Mako do appear to have made some steps to be a little more accommodating however, so some changes have been made which demonstrates their are some modifications made.