All golfers crave more consistency, so this perspective may help you understand why good form may be more transitory than you'd like (and what you can do about it).
Success is a recipe
As we discuss elsewhere, skill in any game area is never all about one thing. It's a recipe of complimentary ingredients. Take for example, the ability to hit standard pitch shots. If you became proficient at this shot, it would have be due to some of the following:
Improved task clarity
A decent process around the shot
A means to problem-solve
The right club for the job
It's unlikely you needed to consciously think about all of these, but as a coach, these are the areas I would consider.
The lag effect
However, any improvement in performance was preceded by learning. So it might have taken a little time to figure out the right recipe for you, and proficiency was earned through effort and applying the ingredients consistently.
Skill will be the reward, but it's rarely instant, it grows organically. So there is usually some lag effect between the effort and the results.
The one downside of improvement
Paradoxically, one of the most common obstacles to improvement, is improvement itself.
When you are discontented with your performances, the cues are that you need to keep working to improve. However most golfers have a satiation level - a point of improvement where they take their foot off the gas. A "That's pretty good for me" moment.
It's a natural response, obviously. The only risk to this response to the first wave of improvement, is that you gradually neglect the things that got you there.
The lag effect in reverse
Neglecting one the ingredients that contributed to your improvement, may not cost you immediately. However, gradually it unravels and you can find yourself back where you started, and scratching your head.
What you must not do
The worst tendency I see golfers do in this situation is to seek a new solution. Jump onto YouTube and mentally download lots of new information, with low chance of relevancy. In learning terms this is a disaster.
The effective response
The more effective response for a golfer trying to recapture form is not to reinvent the wheel, but to reflect on how they have deviated from the recipe that worked.
Coaching for me is a collaborative effort with the golfer. Part of that when forming a recipe that works is that we record it in some way. We are very consciously marking that "this drill is important to you" or "good posture is essential to you"and I reinforce these things frequently.
This trail of breadcrumbs helps identify which ingredient(s) is missing. Often it is simply the work effort dropped and practice waned a little, or something less tangible like a focus of attention that has changed. Either way, we can't rest on our laurels:
Even the most skilled golfers appreciate golf is a perishable skill.
I guess the summary is you get out what you put in. If the performance dropped, then prior to that, at least one of your ingredients went missing.
Don't rip up the recipe and start from scratch, figure out what the missing component was and re-apply. Performance will bounce back even faster, because you've done it before.