It's likely that your potting soil is holding way too much water. While it might appear to dry out on the surface, I bet it's very wet at root level.

have you tried googling PROPAGATION OF SAGE...

based on your very few facts.. it seems like you are stabbing in the dark completely ..

and what are you using for humidity control .. it may make no difference what the media is.. if the leaves can not survive w/o humidity ...

ken

Potting mix isn't very descriptive. Could you be more specific?

I would think sand would generally be too fine but 60% is better than 0%. I would use a mix that is mainly inorganic but of a larger particle size of say the material that falls through an 1/8" screen but not a 1/16" screen. I would use 2 parts pumice to 1 part screened and neutralized peat. When you transplant the cuttings, to what 4inch pots, I would use 3parts screened woody compost to 1 part screened peat to 1 part screened perlite. The salvias generally like well draining soils and "potting soil" doesn't usually fit that description. The sand with which most people are familiar, play sand, is much too fine and doesn't drain well even though it is inorganic.

I also noticed that a lot of time the leaf start browning from the edges in. What does that mean? Should I take the whole plant out? should I cut the part of the leaf that is brown?

Was it brown and crispy? That sounds like water stress, but doesn't necessarily suggest the cause of the water stress. Generally speaking, when you take cuttings that have just rooted and move them to their own individual pot they have to be treated like cuttings again which means lots of humidity.
When in the process is that happening?

Any well drained medium would work. I think ken is probably right and you are not tenting or misting to maintain 100% humidity. You should remove all but a half a leaf, just enough to let you monitor wilting. If tented you will need to vent every day or two to keep from molding. Al

Try a mix of 50% sand and 50% vermiculite. Sage leaves develop mold very easily, so be careful with any misting or tenting.