Leaky plastic slip-fit joints

Hey--just put in a drain in my kitchen. The trap and various tubing is leaking from more than one spot. I can't think what I might have done worng. Tis is my third drain--first one with leaks. Tried teflon tape on one of the fittings--still leaks. (The trap) What else should I try?

Plastic traps or metals traps.

Plastic traps, make sure the beveled end on the plastic washer point towards the flow or water.

joints

Teflon tape is not necessary and it does not help the seal even if you use it. The joints are not tight, the washers are in backwards, or you have bad fittings, are about the only reasons for problems.