As long as you are not regressing you should be pleased. Everyone reaches a plateau from time to time. Don't worry about it.
Your max HR of 167 sounds an awful lot like a 53 year old using the 220 minus your age formula. You can safely forget that you ever heard that formula. Now that you have a monitor you may find that your max is actually higher than 167 bpm (mine is 184 bpm). If you search the forums for heart rate info, you will find a wealth of information to help you measure your improvements.