FM350-2 Counting Problem ('JOB_RD.DONE' staying 'TRUE')

I have a FM350-2 counter card (6ES7 350-2AH01-0AE0) installed on a remote IM153-4 PN IO rack connected to an S7 315-2 PN/DP PLC. I am using this to count pulses, on channel 0 of the FM350-2, from an Endress + Hauser Prowirl 73F Vortex Flowmeter. I have the relevant FM350-2 software installed.

I have followed the set-up guides and examples from this site and also an example project done by a colleague of mine a few years ago with a similar application.

I have set the card up as follows:

Encoders
Signal Evaluation: Pulse & Direction
Hardware Monitoring: Off (Cannot Edit)
Main Count Direction: Up
Hysteresis: 1

Counter Operating Modes
Operating Mode: Continuous Counting
Start/End Value: 2147483647 (Cannot Edit)
Time Window: 10 x 10ms (Cannot Edit)
Use Hardware Gate: NO
Reaction on Closing a Gate: Cancel Count Process

Interrupt Enable
Everything deselected

Outputs
Outputs Behaviour: No Comparison
Substitute Values: All deselected

I have created an instance DB401 for my counter card, referenced this with FC2.

CALL FC 2
DB_NO:=W#16#191
NOP 0


I have opened all software gate by entering B#16#FF into DB401.DBB23.

L B#16#FF
T DB401.DBB 23
NOP 0

I also have a loop counter (DB400.DBW0) set-up to write W#16#64 (100)and W#16#65 alterntatively to DB401.DBB2 (JOB_RD.NO)

L DB400.DBW 0
INC 1
T DB400.DBW 0

L DB400.DBW 0
L 3
<I
JC C1_1

L 1
T DB400.DBW 0

C1_1: L DB400.DBW 0
L1
<>I
JC C1_2

L W#16#64
T DB401.DBB2
JU Cend

C1_2: L DB400.DBW0
L2
<>I
JC Cend

L W#16#64
T DB401.DBB 2
JU Cend

Cend: NOP 0

I am then reading the count:

CALL FC 4
DB_NO :=W#16#191
RET_VAL:=DB400.DBW6
NOP 0

I am calling this FC in OB35(every 100ms). When I go online and view DB401, I notice that DB401.DBB2 (JOB_RD.NO) is W#16#0 despite me writing 100 and 101 to it in my FC400.

Also DB401.DBX3.1 (JOB_RD.DONE) is stuck on 'TRUE'.

I have also noticed that I am getting RET_VAL of '-32621' from FC 4.

Can anyway tell me where I am going wrong? Please let me know if you require more information or a copy of my project.