How to update XML sensor limit ?

Hi, I've written a powershell script to read info on my Dell Equallogic. It request data with SNMP and output a XML as result.

My problem is that the sensor limit are never updated in PRTG. The value is read the first time and that's all.

Example : I have one lun with 1Tb, I've fixed limit warning to 15% and error limit to 5% so XML result will be :

<result>
<channel>Datastore22</channel>
<value>2096760000000</value>
<Unit>BytesDisk</Unit>
<LimitMode>1</LimitMode>
<LimitMaxWarning>1782579200000</LimitMaxWarning>
<LimitMaxError>1992294400000</LimitMaxError>
</result>

If I change the size of the lun, the value is changed and limit are recalculted but only the value is updated in PRTG.

So my question is : it a normal behavior ? is it a bug ? Do I have to change something to enable those updates ?

Thx for help.