Could it be, that you accidentally turned on the mask? You could try pressing Alt + M to clear it and see if you than can sculpt again.


You need to recalculate the normals: in Edit mode, select all and press ShiftN


You want to check Accumulate under the Brush Menu in the top of the 3D view interface while in sculpt mode. Or in the Tool settings in the properties editor : It's the same setting, just accessed from two different places in the interface.

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