There could be a couple of different reasons for this. Don’t mix PVC washers and nuts with metal washers and nuts. Keep them consistent. The plastic washers (ferrules) go with the plastic pipe and washers. The thicker side faces the nut and the thinner side the *cup*. The metal pipe uses metal nuts and square cut rubber washers. Also, make sure that all the pipes fit down into the next pipe as deeply as they can go.
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