Question: Problems with the collect command

Please, can someone give me a hand? I can not understand why in the first case the collect command does not work while the second one works correctly. Many thanks for your attention.trasformaziione_equazioni1.mw
 

esp1 := -a^4+a^2*c^2

-a^4+a^2*c^2

(1)

esp2 := collect(esp1, a^2);

This, apparently, does not work properly

-a^4+a^2*c^2

(2)

esp3 := collect(-a^4+a^2*c^2, a^2);

-a^4+a^2*c^2

(3)

esp11 := -a^2*x^2-a^2*y^2+c^2*x^2

-a^2*x^2-a^2*y^2+c^2*x^2

(4)

esp12 := collect(esp11, x^2);

(-a^2+c^2)*x^2-a^2*y^2

(5)

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