Eddie,
I think you need to read up a bit on how MRP is working. Each time your run MRP the system will schedule according to the requirements at that moment.
So if on the 4th of Feb as you initially mentioned the schedule line date was 06/09/2014 then the system would only have firm requirements up to March 25th.
Everything that is after the 25th can be scheduled out by MRP and as long as the requirement does not fall within the firm zone the system will not keep this schedule line firm.
Based on today's date 22/042014 your firm period runs till 10/06/2014.
So if your dependent requirements keep shifting out and your schedule line does not come within the firm zone you will not get firm schedule lines.
If you want to test change the firm zone to 125 days so that the current date of your schedule line falls inside the firm zone then you will see that this schedule line will get a * assigned next to it and thus becomes fixed. Of course you will have to adapt also the trade off zone as this can not be less then the firm zone.