Which variety are you using? This method will NOT work properly with earlies as they make rapid top growth which then dies back leaving the young spuds. The progressive filling of the container only works with late (main season) varieties which take a long time to mature and grow progressively as earthed up.
If you ARE using a maincrop and it's still going brown, I'd suggest you are overwatering the container. With the cold spring until fairly recently, potatoes have been slow growing and you might have overwhelmed them.

Neither did we, Sodbuster. Tried it once and went back to growing in the ground.

I grow Charlotte potatoes (which I think are second earlies) successfully in plastic tubs. I give them one good watering when first planted, then only a little water each time I earth up (to settle the compost). Once the compost is within 2" of the top of the tub and the foliage is well above the top of the tub I give them more water. I use a bamboo cane pushed down the side of the tub which shows whether the compost is still wet. The tubs have drainage holes about 2" above the bottom so I can tell when the water has reached the bottom. I agree with what Bertie says about the cold spring weather, my potatoes grew much more slowly this year than last so I didn't give them much water until it got warmer.

I'm sorry to disagree. I have been growing early potatoes in bags or about five years and have had great success. I grow Suttons Foremost and start them off in a cold greenhouse until the frosts are over. I feed them with potato fertiliser and water regularly. I place canes down the side of the bags and tie string around them to support the rising shoots (haulms). I harvest them when they have fininshed flowering. Second earlies and maincrop are too vigorous and cheap to buy in the shops. Also more likely to suffer from blight, which the earlies tend to miss.

thank's for the answers I may have been a little to quick topping up with compost.on further inspection the stems are showing classic signs of damping off.