Why does P covariance become non definite when doing Cholesky decomposition for unscented Kalman filters?

Question

I am doing unscented kalman filters for estimation of position and orientation of vehicles.The cholesky decomposition works fine for 8 iterations but then gives me error that P is not positive definite.

At step 8
Error using chol
Matrix must be positive definite.
Error in final_ukf (line 73)
    sP=chol(P,'lower');
1.0e+268 *
    0.0000   -0.0000   -0.0000       NaN       NaN       NaN
   -0.0000    0.0638    0.1927       NaN       NaN       NaN
   -0.0000    0.1927    1.8373       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN
At step 8
Error using chol
Matrix must be positive definite.
Error in final_ukf (line 73)
    sP=chol(P,'lower');