8 year old rescue, we have had her a little over 3 years now. when we first had her, she did everything she was told, straight away. now she has decided that she will try and avoid it like the plague. dont get me wrong, she is still pretty well behaved, but is becoming more resistant if she is told to do something, for example to go on her bed while we are bringing in bags of shopping. mum thinks its because she is getting older, i have an inkling its because she has realised we are not going to beat her for being disobedient like her old owners did. bit stuck.
Behavioural changes like this are much more likely to be a physical ailment. Vet trip is probably in order. Hopefully there is nothing wrong and then a back to basics training regime is your next step.
It is possible that she was obedient through ingrained fear of punishment. If she hasn't been rewarded for this good behaviour since and she now feels more secure, it makes sense that she would start to test the boundaries.
I would be introducing rewards to reinforce her for following the cues - with food, a game or praise, whatever she finds most motivating.
I think Spicy has the answer. She had probably been trained using fear/dominance methods so was obedient for fear of being punished. Now she feels safe and knows you won't harm her she needs lots of positive reinforcement for her to realise it's more rewarding to do the things you want her to than it not to.
I'd go back to basics with clicker training or a marker word (eg "Yesss" said in a particular way whenever she gets something right) and pocketfuls of treats, so you can capture and reward the behaviours you like. Alternatively, as Spicy said, you can use play if she's not food motivated. Ignore the times she gets it wrong and reward her every time she gets it right and she'll soon get the idea.
Eg When she volutarily goes to her bed and lies quietly, you can start to reinforce it by telling her 'settle 'good settle' and if it won't disrupt her and make her get up, dropping a small treat on the bed. If you do this regularly she will start to associate the word settle with lying quietly on her bed and respond accordingly.
Same with things like sitting and waiting for her dinner, offering a sit before she goes through doors etc.
It's a bit like the adult rescue dog version of teenage behaviour, basically boundary testing. As with teenage pups calm consistency and lots of positive rewards for getting it right is the way forward.