I am assuming you have a phase contrast microscope!. Go to your supplier website and download the Manual (if you don't have it) with the instructions to setup your microscope for phase optics. It requires 5 minutes of your time. You might have to repeat this (the alignment) now and then as required.

I just googled "setup microscope phase" while writing to you and I am cpying you the first website found:

http://www.microscope-microscope.org/advanced/phase-contrast-microscope.htm

Regards (and Eid Mubarak)

We would need to know more about the optics and imaging system you are using, it's  less likely to be a phase contrast set up for slice work, but the various forms of DIC all require optimisation too, so Fouad's advice holds up.

are using IR microscope for see the slice.... if not you must used blind patch-clamp protocol

For any optics you have: check the lens of your immersion objective. Carbonate accumulates periodically in a film on top of the lens and reduces the visibility. You might need to rub it off. Check the allignment of your condensor (look online "Kohler illumination to learn how to do it).

If you do NOT have DIC-infrared optics, check the visibility of neurons from young animals: in mice before P12 the myelination is not yet complete and brain slices look transparent. You should see the somas (anywhere in the brain) well outlined. With aging and myelination you will greatly loose visibility. In adult, without DIC optics the cell somas are not visible (you should ask your boss to buy you DIC optics if you work on adult animals... it will save you years of work).

If you do have DIC-infrared optics: Rotate the polarized-lens filter to optimize the contrast of cell somas.

Also

Check that the recording chamber is clean (glas surface free from carbonate concretions). If not, scrub it (lightly acid solution helps) or replace the glass bottom.

Check the contrast settings of you imaging program... but I guess you did that already. Same of course for the light source intensity...

Good luck!