My experience this is caused from one of two things:
I - fundamentals change you aren't aware of - if you stop making everything then it could be head position or eye position.
Shoot some straight in long shots with no english and see if you are making them or if you have to move your head to see them correctly. That will give you a clue. If you're missing them the same way every time your head/eyes are probably in the wrong place. (Because you said you're still setup the same) If you're spraying it and rattling it and missing on both sides, you probably have introduced some wobble somewhere. Could be bad pre-shot routine, bad rhythm, bad grip. Work with someone to figure it out. As suggested Scott Lee is excellent.
II - You watch better players and start using english too much to try and get position. This is usually because you're overthinking position in the first place.
Throw some balls out and just shoot them in without english. Focus on using ball speed FIRST to try and get position. And then focus on getting the right angle so you can get position on the next shot without using english. You'd be surprised how many position shots you can get to with only top and bottom.
I suspect when you start shooting balls without english you'll start making a ton more. Then, even in matches, don't use english other than top/draw unless it's an emergency.
At this point in your game, every time you use side spin it should be a conscious choice and one not to be made lightly.
If you are making balls like crazy without english, wait, wait, wait to incorporate it back into your game. Practice specific shots with specific english until you have them as second nature and pay close attention to where you have to aim to make them. Only after a particular shot/english combination is a sure thing can you incorporate it into your regular game.
For example, making a slight cut into the corner with outside bottom english to come back to the side of the table you're shooting from is a useful shot - So practice it from all different angles, lengths, etc and pulling back to specific points on the rail. Once you've mastered it and it comes up, hit it with confidence. Then pick another shot - maybe working off the end rail from a side pocket shot etc...