[SalesForce] apex:actionFunction not passing the parameters

I am currently trying to call an Apex controller function from an apex:actionFunction, but I can't seem to get the parameters to be passed to the controller function.

ActionFunction

<apex:actionFunction name="Sign_Up" action="{!SignUp}">
  <apex:param id="deliveryId_signup" name="deliveryId_signup" value="" assignTo="{!deliveryId}"/>
  <apex:param id="slotType_signup" name="slotType_signup" value="" assignTo="{!slotType}"/>
</apex:actionFunction>

Button

<button type="button" class="btn btn-primary btn-block" onclick="Sign_Up('{! deliveriesDateMap[date][school][delivery].delivery.Id }','Signed Off');">                                                              Sign Up                                                                         </button>

Apex Function

public String deliveryId { get; set; }
public String slotType { get; set; }

public PageReference SignUp() {
    //I have tried to also get the params using the options below, but no success
    String passedParam1 = Apexpages.currentPage().getParameters().get('deliveryId_signup');
    System.debug(passedParam1);
    String passedParam2 = System.CurrentPageReference().getParameters().get('deliveryId_signup');
    System.debug(passedParam2);

    try{
        Contact c = [SELECT Id FROM Contact WHERE Id =:contactId];
        Delivery__c d = deliveriesMap.get(deliveryId);

        Attendance_Slot__c slot = new Attendance_Slot__c (
            Delivery__c = deliveryId,
            Type__c = slotType,
            Contact__c = c.Id
        );

        insert slot;

        successMessage = 'You have successfully signed up for the delivery';
    } catch(Exception e) {
        errorMessage = 'There was an error signing you up for the delivery\nError: ' + e.getMessage() + '\tLine: ' + e.getLineNumber() + '\t' + e.getCause() + '\t' + e.getStackTraceString();
    }

    return null;
}

Any help trying to understand why are the fields not being populated on the controller function would be great!