Multi Select Text Field

You can likely simplify the formula to 

if( ToText([Suburbs Field]) = "", "N/A", whatever the other condition would be ). 

To cover your point though it likely has to do with confirming that something is actually happening on the form for the form rule to fire. The alert pop up form rule will only trigger when someone is changing something on the form and something is actively changing on to trigger the rule. With that - you also need to make sure that: 

1. All of your fields you need for these rules to fire are on the form, either in view or on an admin/config tab.
2. Your rules are sequenced correctly so that they fire in an order that they trigger / read from each other
3. Make sure you're not caught in a trap where nothing is changing on the form to make the rules fire. 

From what I understand in your comment it sounds like: 

• User opens form and either the suburbs field is blank or they change that field to blank and the response is 'N/A'
• When the suburbs field changes to N/A - form rule pushes N/A to another value
• You want a pop up at this point

If thats accurate - you want to make sure that bullet (3) with the pop up is the last rule to fire - else the conditions that make it true haven't processed yet (the value being N/A). You also want to make sure that the conditions are actually changing in a way that the user is changing a value or a form rule is setting something like you said and then tying that to your rule to fire the pop up.