Cisco C3750 and PRTG netfllow

Question

I have a core switch (backbone) of our network that broadcasts all the vlans to other switches. I am trying to configure netflow on the switch and link it up to a Netflow sensor on PRTG.

I followed this implementation: https://www.nazaudy.com/index.php/12-technology/cisco/34-enable-netflow-v9-in-cisco-switches-with-prtg

For some reason when i do show flow export statistics, shows '0" as if nothing is getting exported!

This is my stacked core switch type: C3750X, IOS Version 15.2(4)E5, License Level: ipservices, sdm: "desktop routing"

This is the configuration i am using to get the netflow on the switch, (i tried with v9 but same issue, the below is v5):

flow record PRTG-FLOW-RECORD
 description Record-Network-Traffic
 match ipv4 tos
 match ipv4 protocol
 match ipv4 source address
 match ipv4 destination address
 match transport source-port
 match transport destination-port
 collect timestamp sys-uptime first
 collect timestamp sys-uptime last

!
!
flow exporter PRTG-EXPORTERv5
 description Export-Flow-to-PRTG
 destination 10.7.7.200
 source Vlan7
 transport udp 9997
 export-protocol netflow-v5
!
!
flow monitor PRTG-MONITOR
 description Monitor-PRTG-Flow
 exporter PRTG-EXPORTERv5
 cache timeout active 60
 record PRTG-FLOW-RECORD
!
!
sampler PRTG-SAMPLER
 mode random 1 out-of 32

When i do show exporter statistics:

Flow Exporter PRTG-EXPORTERv5:
  Packet send statistics (last cleared 05:45:07 ago):
    Successfully sent:         0                     (0 bytes)

  Client send statistics:
    Client: Flow Monitor PRTG-MONITOR
      Records added:           0
      Bytes added:             0

When i do show flow exporter:

Flow Exporter PRTG-EXPORTERv5:
  Description:              Export-Flow-to-PRTG
  Export protocol:          NetFlow Version 5
  Transport Configuration:
    Destination IP address: 10.7.7.200
    Source IP address:      10.7.7.1
    Source Interface:       Vlan7
    Transport Protocol:     UDP
    Destination Port:       9997
    Source Port:            49747
    DSCP:                   0x0
    TTL:                    255
    Output Features:        Not Used

When i do show flow monitor:

Flow Monitor PRTG-MONITOR:
  Description:       Monitor-PRTG-Flow
  Flow Record:       PRTG-FLOW-RECORD
  Flow Exporter:     PRTG-EXPORTERv5
  Cache:
    Type:                 normal
    Status:               allocated
    Size:                 128 entries / 6660 bytes
  Cache:
    Type:                 normal (Platform cache)
    Status:               allocated
    Size:                 Unknown
  Timers:
                       Local        Global
    Inactive Timeout:     15 secs
    Active Timeout:       60 secs      1800 secs
    Update Timeout:       1800 secs
    Synchronized Timeout: 600 secs

I am applying these two commands under the interface VLAN:

ip flow monitor PRTG-MONITOR sampler PRTG-SAMPLER input
ip flow monitor PRTG-MONITOR sampler PRTG-SAMPLER output

For some reason i get the foolowing if i apply it under a Gigabit interface:

Error: Flexible Net Flow is not supported on this interface

My PRTG server under port 9997 is open, i made sure of that! It's just not receiving any netflow data

I would appreciate any help at this level!