Is your macerator enclosed within the black tank,or is outside?(like installed as an accessory).
I just installed a macerator pump and had problems,which I posted aboot here on this forum. Mine was acting the same as yours. I took off the 3 inch hose where it connects to the macerator and could see the blades were clogged up solid with some sort of cloth. I took off the four bolts and pulled the cover off and cleaned out the blockage. It worked great until..
I started having the same trouble again. This time it was only about 15 seconds between the pump being turned on and the FUSE BLOWING. I took it apart once again, and, yep, another large cotton rag clogging up the works. It came apart again and cleaned out.
Word of caution: (and I m assuming you have a Flojet macerator).
When tightening the four brass castle nuts that hold the cap on, do not use much torque when tightening. I snapped off one of the 4 brass rods that held the cap on. Brass cannot take much torque. I replaced it with a cheap 3" length of allthread from the hardware store. I coated the all thread with grease before installing it with hopes it will prevent rust. I now have 8 spare rods.