One of those 'A-ha!' moments

Over the years I have been stymied by the learning process of some of my students- two of my current students have provided me with an epiphany...now I am working out how to address the issue.
What I have noticed
Like any other teacher, I have a wide mix of abilities in my classes- all kids come to Spanish, regardless of whether they are learning disabled, autistic, have speech impediments, etc. Making these kids feel successful is extremely important to me yet it is a continual challenge, particularly as they go through the grades to find those successes. As I mentioned above, two of my current students have provided some insight. Both of these girls are learning disabled...both of them have a great accent in Spanish and both are able to easily and creatively use a wide variety of phrases such as please, thank you, you're welcome, bless you, "how do you say ___ in spanish?", hello, goodbye, etc, yet when faced with a question like "How are you?" or "What color is the ___?" they struggle to come up with an answer. One moment it seems like they are extending and another moment they have great difficulties accessing class. This has puzzled me to no end. I know they both have memory retrieval issues and have challenges recalling information, yet they were using lots of phrases and words appropriately. What was the mystery I was missing?
What I suspect
I believe the solution is in fact simple- situations where only one possibility exists are learnt and recalled better than situations which are open ended and have multiple possibilities. In other words, there is only one word to express "please", but there are many answers to the question "How are you?" (my students have a choice between 12 possibilities by the end of 2nd grade). I suspect these girls' brains are able to pull up and use the vocabulary that falls into the first category- 1 word/phrase for a situation- but can not sift through a set of multiple possibilities in order to choose a specific word/phrase to use. Perhaps their brains are unable to store this information in a manner which allows them to access it later, or the process breaks down before it even gets to the part of the brain where this information is housed, or they can not pick out from what is stored the particular word/phrase they want to use. Certainly amount of usage of vocabulary plays a part- however, we use expressions of courtesy and the question 'how are you' every class, so they are used in equal parts.
Going forward
The challenge for me now is to figure out how I can present vocab for these types of kids (because I have many like this, not just these two girls) in a way which mimics the '1 word/phrase for a situation' phenomonen while at the same time integrating it into a 'multiple possibilities' framework. I shall ponder this further....