Perhaps the plant is having to give up most of its nutrients to the flowers at the expense of the leaves? Do you feed your alstroemerias? I know that they are quite hungry plants and partial to lots of fertile organic matter.
I keep giving my alstroemeria (a small 30cm high pot-based variety) a liquid tomato feed once a week and it's doing splendidly. Is yours in a pot or in the ground?
I'd give it a tomato feed then Marc - as it's liquid based, it gets to work pretty quickly and you should notice an improvement within a couple of weeks. The tomato feed will also promote more flowering.