A few thoughts/guesses, without seeing what the prints look like before fixing, it's hard to guess.
The color changes ( from lavender or purple to brown ) in the fixer, and this can change your perception of contrast or depth. Are your white highlights still bright white? If the highlights start to get tone it can dramatically reduce the apparent contrast.
There is not very much silver chloride on the surface of a salt print... it really does not need strong fixer. I use about a teaspoon or 1.5 teaspoons of hypo in 60-100ml of water for about 3 or 4 minutes ( that's much less than a 10% solution ). ( When I'm being careful on a print meant to be a final version, I fix 2m, then 2m again with fresh solution ) I usually do add a pinch of sodium carbonate to the water before mixing the hypo, but I've done it many times with and without alkali and I have never seen much difference in "bleaching". I have seen some bleaching start to appear when I fix for more than about 4 minutes.
Here is a suggestion:
Expose your salt print in indirect sunlight until you just see some tone starting to show in the brightest highlights. Then hold it in full direct sunlight ( point it right at the sun! ) for 1.5 to 2 minutes ... this "locks in" the darks and gives "depth of printing" which will make the print easier to tone if you use toner and should make the darks hold up better in the fixer.
If the darks are not dark enough when the highlights start to show tone, it will be difficult to make a good print. If this is happening, try taping a piece of plain white paper over the glass of your printing frame ( this is called a "diffuser" ). With the diffuser in place, try exposing to indirect sunlight for a couple of hours first: you want to see only the darks and they should reach about 40 or 50% of where you want them to be... then do the rest of your exposure as usual. The diffuser gives the darks a head start so that they are partly printed in before you go ahead with your normal exposure. In this way it is possible to print different negatives that have different "contrast" ( really, density range of the negative ).
A lot depends on the negative!
Good luck and have fun!