I have a similar situation with Hungarian language. My passive skills and vocabulary are very nice, but I still have problems with writing. My recommendations:
1.If you have some topic, just write on it! Don’t afraid of your mistakes and try to write in plain language without too complicated grammar structures. However, if you have a topic, you haven't the problem itself... So I will write about other methods.
2.If you haven’t a topic, just write about language itself. For example, in Hungarian I wrote a text about the word «magyarázni» (to explain). I find this word very unusual because it derived from the word «magyar» (Hungarian). I made a lot of mistakes, but the text itself was quite interesting. I think any language have many interesting words, grammatical nuances and expressions.
3. If writing about languages doesn´t work for you, just write something even if it looks not very meaningful. I mean something like this:
«I am learning Czech and I would like to write something incredibly interesting. Unfortunately I haven’t any fascinating idea or unforgettable experience which can be described in my text. Therefore I am writing spontaneously now. I am trying just to write something, rather than to narrate an amazing story. However, I think it’s acceptable for my writing practice, if my text isn’t too prolix or meaningless…»
As you can see, this method works and surprisingly, you even can use many vocabulary words this way. Method №3 can be used only occasionally.If you write too many texts that way it will become boring for your correctors :)
[Upd. My examples in Hungarian are not that good and my idea can be understood without them.]
My comment is long, but I hope it really helps you.