Fast and Slow Blow Fuse
SMPS usually use slow blow fuse and if you replace it with a fast blow fuse (with the same ampere rating) chances is high the fuse will blow again. It can blow immediately or after using the equipment for sometimes. The reason you do not put back a slow blow fuse is either your compartment have no more stock or you wat to save the money. A slow blow fuse price is very much higher than a fast blow fuse.
If your compartment totally do not have anymore slow blow fuse, you could use a fast blow fuse for testing but with atleast 1amp higher than the original one. I have seen people use about 2 amp higher. This mean if the slow blow fuse is 3 or 3.15 amp they replace it with a 5 ampere. Do it only for testing purposes and not for permanent usage.
Remember, when replacing a fuse be it a slow blow fuse or fast blow fuse always use back the the same type and with original rating. I do not want you to face any legal action just incase if you have repaired an equipment and the whole equipment burn down in your customer place. Even when you sleep you will have a peaceful night and need not worry if the equipment will burn.