My lHO layout is 4x6 and i'm using 22" radius curves and my passengers wont couple. Should i us a larger radius curves.
22" radius should work fine. The couplers themselves are probably the problem. Try replacing them with Kadee #5's or #148's.
Usher,
First of all, with a 4x6 railroad, 22 inch radius is as large as you can go. Remember, that measurement of the radius is a track centerline measurement so you basically have no room left.
As for the cars, how long are they, and who makes them. If they are Walthers, there is a good chance that the diaphrams at the ends of the cars the real culprit. A friend and I discovered that you can put a long shank Kadee coupler on one end of each car. Attach that to the shorter shank coupler of the next car. This creates a larger distance between the diaphrams but it is not unsightly and the cars will negotiate a tighter radius than before and the couplers will hook together.