The pawl is engaged in the teeth of the ratchet by a spring under the button on the end of the lever. If this button has unscrewed enough it may not exert enough pressure. It is possible that the spring is broken or missing, and also possible that the pushrod has become disconnected from the pawl, or the mechanism is rusted so stiff that the spring cannot move it. 
Removing the under seat panel should enable you to see what is happening or not happening. There is a slight possibility that the pawl or the ratchet of both are just plain worn out, although this would indicate decades of abuse (not holding the button while pulling on).
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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