I have POA but I haven't really exercised it in that I already liaise with doctors/carers so at this point it isn't offering anything extra. I can't use it to force my mum to receive treatment.
Oh but it does offer something extra. The whole point is that you can actually consent on her behalf as though you were her, if and when she does not have capacity to make a decision. If she doesn't have capacity for whatever the decision is that needs to be made, at the point it needs to be made, it's up to you. Legally, it's YOUR responsibility to be content that you are acting appropriately within your remit as POA, and there are legal mechanisms for HCPs, other professionals and concerned family or friends to challenge if they think you are not. Yes a capacity assessment could be useful but it's not a legal requirement, and often capacity is variable so a snapshot assessment isn't going to give all the information needed to decide. Consult a solicitor who specialises in this, if that would give you confidence, and then advocate for your mother and yourself as the POA.
Healthcare professionals often haven't had a lot of training on the Mental Capacity Act 2005, we're forever having to set HCPs straight especially if they don't understand dementia or certain other conditions very well. It's also very hard for them when they don't know the patient and are rushed off their feet. They usually don't argue back because we have made sure that we know our stuff and can articulate it, although sometimes they go off 'to check' and then come back with a different attitude. It is of course a good thing that they do check if they are not sure. Although having to rexplain umpteen times is very tedious.
It's very important for POAs to be lodged on file, even if they are are not yet applicable or only applicable some of the time. But the NHS is very flaky on POA. They should ask regularly and get that on file, as should social care, charities involved with the patient, etc. But we've been asked approximately twice which is about 100 times fewer than we should have been (given the different parts of the system that are involved). And I did have a polite but longwinded argument with a charity not long ago who just would not or could not 'get it' (and also couldn't grasp the different types of POA). So you have to be proactive.
Make sure that your mother is also being given information in a manner, and at a speed, that is appropriate for her, whether she is going to be making the decision perhaps with some additional frameworking from you, or whether it's you making the decision, and is treated with respect even when you have made a decision for her. And that she and your father has everything above the very trivial written down by the HCP to be referred back to. It's such a struggle for us to get that done properly too, it makes me quite cross. (See Accessible Information Standard 2016)