Advice/thoughts/reasons please

I've recently been doing the mother in laws exterior woodwork painting, rub
down, undercoat and top coat on facia, guttering/downpipes, window frames
and garage doors with brushes, not rollers.

Although it's taken a long time and it looked top-job when completed, a
fortnight later the topcoat has developed miniscule bubbles, about a quarter
of a mm across over all the surfaces. I'm led to believe that this is
normally due the undercoat not being dry before coating, but I can pledge
that it was... 3 or four days in some cases, 24 hours in other cases.

There is a slight twist to this tale though.... MIL wanted a two tone colour
scheme, magnolia and green. The magnolia hasn't bubbled, the green has.
The green was a Dulux mixed product bought from the local shed, along with a
mixed undercoat to match. Could it be a fault in the mixing process? Am I
being paranoid?

I'm left with the God awful feeling that I'm going to have to recoat it all
again (come better weather)... bless my mother-in-law ;-(

Russ