CTR (conversion) is the percentage of the number of people that subscribed on you (only those people who were engaged by the service) over the total number of touches (to total coverage, i.e. to the amount made by the service likes, subscriptions, comments).
Good conversion is considered to be promotional with a CTR of 5-6% and above.
Disconnect the promo with a conversion rate below this level, and continue only with effective promo.
How to increase a conversion rate?
If you run 3-4 promos, and in day or two see that the conversion level is low - remove existants and add new ones. If after that it remains low - work on your content fullfilment. Capture high quality photos, good description of the profile. Try to do your account interesting for users.
Customer support service by UserEcho