The ottoman empire in the second half of the 1300's completely surrounded Constantinople, how did it take 80 years for Constantinople to fall even though it is completely surrounded?

I answered a similar question recently:

Why did the Ottomans only invade Constantinople in 1453 and not before?

Basically, there were several reasons. Ottoman territory wasn't completely stable and they had several other issues to deal with (wars in the Balkans, rebellions in Anatolia, etc.). They also did try to conquer Constantinople in 1422, but that time they were defeated. After that they were content to make a treaty with the Byzantines and simply contain them, rather than attacking. Constantinople probably could have lasted a lot longer that way, so really, the question is, why 1453 and not later? In 1453, Mehmed II was a new sultan, very young, and felt he had to do something big in order to avoid being challenged by other claimants.