Hi, are all the leaves yellowing or just the older ones? If it's the older ones I wouldn't worry - this is natural as the older leaves die off to be replaced by younger ones. The nutrients from the dying leaf will be going back into the plant, a bit like when you leave daffodil leaves to die off. When the a leaf is totally yellow you can cut it off.
Phormiums are tough and will usually cope ok with wet conditions and respond well to a bit of 'benign neglect'. I know of a 'plantation of phormiums' that have survived totally un-nurtured in the damp and shady quadrangle of an art school for years and years and years, and they flower every year.
As for flowering - they usually flower best when the roots are a bit constricted, like agapanthus. Try a potash-rich feed next spring to see if that encourages them .