There could be various issues, depending on the type of machine you have. Contact the manufacturer's representative first to see if you can troubleshoot the problem over the phone. Failing that, you have have to have someone come out to look at it.
If you are using a humidifier bottle with your machine (the unfilled type that you have to refill yourself when empty), then it's possible that you are filling it too full to where the water is bubbling right into your tubing. If so, try reducing your water level. Also, if your machine sits too close to the wall it heats up the body of the machine and the water in the bottle. It can condensate the water into your tubing. Keep your machine at least 12" from the wall.
There is a community of oxygen concentrator users that are willing to give you their opinion about the products. They can be found at http://www.oxygenconcentrators.org/. The website also compares different models of concentrators.