Thank you for your question. It is difficult to diagnose a plant problem and advise without a photo or an examination of the plant itself. Are you able to bring a stalk or two to the CC Extension office? We have Master Gardeners there 9-12 and 1-4 weekdays. 200 Warner-Milne Road, Oregon City. Thanks!
Another expert suggests the stems may be old and woody, which is a condition of older plants. Since you did not indicate the plants’ age, this is hard to know. Perhaps you’ve already been to the Extension office....