Problem with determinant calculation [closed]

I have a 12x12 matrix (2D list), called M, which depends on some variables. Usually I set all of the expect of one (call it x).

To my surprise, when I use M1=M/.{x->1} Mathematica (8 as well as 9) completely ignores statement and does not replace x by 1 in M1 matrix. Moreover, when I use Det[M] (without replacing x by a number), I cannot get any result. I have to set eg. x=1 and than calculate determinant for this value, which is not very practical for the further calculations.

Do you have any idea where might be the problem? I do not think that my matrix is so complicated that Mathematica cannot simply calculate a general expression for its determinant. I do not use Simplify or FullSimplify.

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Can you share the code you are working on ? –  Sektor Jul 30 at 9:29
The matrix is coming from an external file, which is exported from another notebook. It seems to be OK. Its components are nonlinear complex expressions. In the code I just read the matrix and try to calculate its determinant. –  LLapsus Jul 30 at 9:42
You can have a look at the internal representation via FullForm[M1]. This could give you a clue why it behaves the way it does. –  Thies Heidecke Jul 30 at 10:11

closed as off-topic by Kuba, Yves Klett, Verbeia♦Jul 30 at 13:33

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Definitely, I found what went wrong. It was just a minor problem. I read my matrix from an external file but Mathematica saved i as a SparseArray. I just learned that ReplaceAll does not work with sparse arrays. When I convert it to a Normal form it works.