davorg provided you with a working solution, let me explain shortly why your code is not working.

Paraphrasing your code into pseudo-code:

if pattern matches
   set variable $fno   based on above pattern match
   set variable $figno based on above pattern match
   substitute pattern with string that uses $fno and $figno
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so the variables $fno and $figno are set once based on the if-match (which grabs the first occurence on a line). Then the substitution replaces all (/g) occurences of the pattern with a string which contains the two variables. These variables don't change during the substitution, therefore you always get replacement according to the first match. The only variables that change 'automatically' during the substitution are the special variables $1, $2, ... as you can see in the result string ('Figure 2' is correctly replaced for the 2nd occurence).

There is nothing wrong with your patterns, both work just fine - with the usual caveat about parsing such markup with regexes ...