DIAGNOSIS This is almost always a heat input issue. Your burner isn't supplying enough continuous heat to maintain the boil once the initial pressure is reached. The large mass of jars and water requires sustained energy to keep producing steam.
FIX First, don't panic and crank the burner to max—that can cause spitting. 1) Give it a minute. Sometimes it takes a little adjustment to find the sweet spot. 2) Increase the heat slightly until you hear the regulator weight begin to rock steadily again and the gauge climbs back to your target pressure (e.g., 10 or 15 PSI). 3) Once it's steady, note the burner setting. That's the "maintaining" heat level you'll use for the rest of the processing time. On many stoves, it's significantly lower than the "bringing to pressure" heat level.
IF THAT DIDN'T WORK Your stove might simply lack the BTUs needed for a 23-quart canner full of jars, especially if it's a smooth-top electric or an older coil model. Try using a smaller number of jars to reduce the total thermal mass, or consider using a different, more powerful burner if you have one.
HEADS UP The processing time doesn't start until the canner is at your target pressure. If the pressure drops below your target during processing, you must bring it back up and restart the timer from zero. Consistent pressure is key for safe processing.