Issymum, I have learned that certain types of questions will get a stonewall reaction from dd, while questions worded in other ways will often elicit a more detailed response - ie, more than a single word!
I have a copy of the weekly schedule, so for instance, on Wednesday I can ask about gym, and what games they played and who she played with. On Thursday, I ask about assembly (who got the stars or if they sang etc), or on Friday I ask about what they did in the computer room or who brought what for show and tell. By asking in a leading way, or a way that implies knowledge of her school routine, she often will answer more than if I simply say "so what did you do today".
Sometimes I'm really silly with her, and say something like "did the pink elephant come to ballet today" or "someone told me your ballet teacher was wearing a clown costume today!" She of course, laughs and then often will proceed to tell me what did happen....with a bit of prompting, of course!
I know who she sits next to, so I can say "what did Isabelle have for lunch today" or "did you play with Grace during break today". But the question almost guaranteed to get her talking is "was anyone naughty today?" She then will chatter and tell me about it.
It also has helped a great deal to have a friend whose elder daughter was in reception with the same teacher a few years ago. That friend guides me as to what goes on in the classroom/with the teacher, and what to expect. It helps fill in the many gaps left by dd's patchy responses!
I'm sure you've tried most/all of these tricks, so it may be that your dd is more determined in her silences than my little chatterbox!