Mannequin is giving me issues all of a sudden

IDk why, but now when I’m walking the mannequin, if the the height is above him, he will touch the ground but when it drops below his current height, he keeps walking at the level, So it’s like he’s walking on air & when I stop controlling him, he goes from run to slow walk & just keeps walking unless the terrain slows him… It has been working fine for a week, but all of sudden, he starts doing this & I’m super new so, I don’t know if it was something I accidentally hit or how I can fix it.