If the lower plug-in connection rattles on three-piece paddles that have to manage without an additional clamp, neatly glued strips of smooth, thin and preferably stiff adhesive film can help. Stick it on, try it out - strip by strip - until the tube just goes in.
High-gloss carbon and aluminium are often simply too slippery to grip securely when wet or with sweaty hands. But there are various options for perfect grip even on the slipperiest of sticks.