Usually temperatures stay roughly within the same range from cycle to cycle if there are no changes in your environment or sleeping conditions. Slight changes in your environment or sleeping conditions, however, can impact your temperature range from cycle to cycle. Changing your waking time, using heavier or lighter blankets, turning on (or off) the air conditioning or heat, or seasonal changes can change the range of your temperatures. As long as you can still see an ovulation pattern on your chart, the actual temperatures are not critical.
If you notice significantly higher or lower temperatures that you can not associate with a change in your lifestyle or environment, then this is something to mention to your doctor, especially if it is taking a long time to conceive.