Way back in the 140 series, with enough force the black plastic latch housing could sometimes get pushed down on the supporting shaft and make for a latch failure. If you pulled it hard straight up it would usually work again, but if it moved too far down it was game over, something inside became dislodged. There was a recall where dealers put a little clip on the supporting spring coil so it couldn't move down. I thought the later 740 ones were molded on the shaft so they couldn't do that, but if yours was somehow able to slip down that may be what caused the latch failure and you may not be able to fix it.

You've liklely also tried peaking inside with a good light for debris like hair, picking with tweezers, poking around a bit inside and tapping it hard upside down then suctioning with a vacuum, finally using compressed air or spraying in some WD-40 to see if something's stuck and preventing full travel and the latch catching. A bent or broken spring down inside would likely mean the end.

The fronts also have either a microswitch or some kind of electrical contact to know when the belt is latched and not activate the chime when the key goes into the On position. A failure with that sensing mechanism breaking could also be getting in the way of the latching mechanism. Some also have a tiny bulb inside for illumination that's connected to the dash lighting, so those also have wires inside that could get in the way.

The red plastic tabs get old from being in sunlight and more easily wear through or get broken by the belt tab. I got hold of a used rear middle seat latch for cheap that had two good red tabs so I'd have a spare. The latches were different than mine, but the red tabs were identical
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now