The main function of the round recess is to locate the coin for the weight check. The diameter of the recess is at the maximum allowed - according to the issuing mint - for a coin. A coin that is at that maximum will be a tight fit. The recess of each Fisch is checked with a disc at the maximum diameter. 

The diameter of the recess and the length of the slot of the Fisch is the same. A disk at the maximum diameter (and thickness) is used to check the slot of each Fisch, too. Therefore if the coin passes through the slot you need not be concerned if it is tight in the recess. 

With gold coins, the thickness, diameter and weight checks are the most critical. If a coin fits through the slot it is not oversize. If a coin tips the Fisch it is not underweight. Pass both those checks and the coin is not a common metal counterfeit. Refer to the Fisch Principle. (Obviously, a coin that is so far off round that it is oval wouldn't fit in the recess at all for the weight check). 

Coins tight in the recess is particularly common with the smaller 1/4 and 1/10 oz gold coins. It is not unusual that the coin sticks in the recess. A good way to get a coin out is to slap the Fisch with the coin in it against your palm - Fisch with coin to palm so when it dislodges it is in your hand.