I'm sure this topic has been covered many times before but i can't seem to find the answer that settles my nerves. My transom drain has the twist lock or snap in style. I use the twist lock inserted from the outside. I notice this year while getting things ready for the season that my plug looked cracked. I went a purchased two new ones, when I install one of the new ones it will never tighten up enough that I can't pull it out. When I try the old one it will go in and tighten up that I cannot pull it out. What am I missing? Can the tube be that worn that the new, softer rubber won't grab? I've been considering installing a treaded type drain just so my mind is at ease. Help?