Parameter store=True defines such behaivour... 
to be precise... it computes the stored value once, store it in db field and never recompute again.. 

In order to test this behaivour, first remove store=True param... and try changing values ( it should recompute on each view reload...)
refer to documentation for more info on subject

hope it helps

edit1:

Parameter store = True, can accept a list of ids to recompute.. , so you should make store=_check_to_recompute

and def _check_to_recompute(self,...):

   return [ids]

returning any list of ids to store parameter of computed field will trigger recompute on those fields, and making store=_some_method is basicly store != False , so the data will be store.. .
look at documentation for v7 .. might be more explanational... also look for new api documentation on link above

EDIT 2.

Look at account_invoice example.. 
    @api.one
    @api.depends('invoice_line.price_subtotal', 'tax_line.amount')
    def _compute_amount(self):
        self.amount_untaxed = sum(line.price_subtotal for line in self.invoice_line)
        self.amount_tax = sum(line.amount for line in self.tax_line)
        self.amount_total = self.amount_untaxed + self.amount_tax

according to this, _compute amount is triggered whenever invoice_line.price_subtotal, or tax_line.ammount is changed...
maybe go that way.. try to debug/test on account_invoice model for beter comprihesion of this subject