Unable to edit cash flow in Interest Rate Instrument

Hi Experts,

I am facing a strange issue while trying to change Cash Flow in Int. rate instrument.

Scenario:

I am trying to edit a flow in FTR_Edit and system is allowing me to change most of the flow. Please see below:

But if try to change the very next flow on the same date i.e. 960.93 USD Edit Flow option is NOT coming at all.Please see below

These flow has NOT been posted to accounting still.

Any one has any ides/suggestion why system is behaving like this.

Cheers !

Adi