Can my crowns be removed before they fall off and be glued again?

Well, if you are noticing that your crowns are coming off.  I would immediately go see a dentist to have them assessed.  It sounds like you may have open margins.  This could lead to cavities on your exposed services and cause you to need a root canal or even loose your teeth.  Believe me, it would be much better for you to go to a dentist and have those crowns removed, and then new crowns placed in their places.  For crowns that I place on my patients' teeth, I expect them to last at minimum 10 years, depending on a patient's personal oral hygiene.  Let me know if this answers your question. 

Well, obviously you should see a dentist.  Usually if the crown is loose, it will come off easily.  However, it is also very possible that your have had some gum disease and your gums and bone have receded, and the less bone support the more the tooth will feel "loose".If there is a gap under the crown, then there is either cavities under the crown or the crown didn't fit well and the cement washed out over time.A well fitting crown that comes off can usually be recemented without any problem.However, if it is ill fitting and/or there is decay, then you would more than likely need a new crown.