how do I extract variables from a service callout to assign them to the response?

[#1]

I want to callout a service, extract the variables from that service's json and assign those variables to the response of this flow. The service callout's response contains the variables, but I am unable to extract them. What can I do to fix this?

Policies and final response below

service callout policy:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ServiceCallout async="false" continueOnError="false" enabled="true" name="refresh-external-callout">
    <DisplayName>refresh external callout</DisplayName>
    <Properties/>
    <Request clearPayload="true" variable="myRequest">
        <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>
        <Set>
            <QueryParams>
                <QueryParam name="grant_type">refresh_token</QueryParam>
                <QueryParam name="refresh_token">{oauthv2accesstoken.generate-external-token.refresh_token}</QueryParam>
            </QueryParams>
            <FormParams>
                <FormParam name="client_id">{request.formparam.client_id}</FormParam>
                <FormParam name="client_secret">{request.formparam.client_secret}</FormParam>
            </FormParams>
            <Verb>POST</Verb>
        </Set>
    </Request>
    <Response>calloutResponse</Response>
    <LocalTargetConnection>
        <Path>/am/oauth2/token</Path>
    </LocalTargetConnection>
</ServiceCallout>

extract variables policy:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ExtractVariables async="false" continueOnError="false" enabled="true" name="extract-refresh-callout-response">
    <DisplayName>extract refresh callout response</DisplayName>
    <Properties/>
    <JSONPayload>
        <Variable name="access_token">
            <JSONPath>$.access_token</JSONPath>
        </Variable>
        <Variable name="refresh_token">
            <JSONPath>$.refresh_token</JSONPath>
        </Variable>
        <Variable name="expires_in">
            <JSONPath>$.expires_in</JSONPath>
        </Variable>
    </JSONPayload>
    <Source clearPayload="false">calloutResponse</Source>
</ExtractVariables>

assign message policy:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="external-token-response">
    <DisplayName>external token response</DisplayName>
    <Properties/>
    <Set>
        <Payload contentType="application/json" variablePrefix="@" variableSuffix="#">
            {
                "access_token": "@access_token#",
                "expires_in": "@expires_in#",
                "refresh_token": "@refresh_token#"
            }
        </Payload>
    </Set>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
    <AssignTo createNew="false" transport="https" type="response"/>
</AssignMessage>

response I get:

{
    "access_token": "",
    "expires_in": "",
    "refresh_token": ""
}