Car insurance, confusing?

I had a Vauxhall Astra and had fully comp insurance, I then sold the car and bought a ford mondeo the same day, I contacted my insurers and asked them to transfer the details over (i have done this previously with a different insurance company). After giving them all the details, i was told that they would not be able to insure my car, i asked why but they couldn't give me an answer. Subsequently i cancelled my insurance (so did the insurance company) and have insurance with a different company. Can anyone explain why they could not insure a ford mondeo even though i had existing insurance with them?

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