I think a lot of teachers have that problem. Most students find that when they play stuff they already know but stumble through bits they can't, that is practicing. I tell my students when you practice it all should be only parts you can't play.
I make sure that a students understands that when you sit at the piano instantly you must ask a question. Am i playing for fun or am i studying? If you are studying then you don't play what you can you work on the sections you cannot. In fact i told one student who couldnt get this idea to work for them, never play what you can, only play what you cant for 1 week. When practicnig it shouldn't be comfortable, it should be a tough struggle to memorise, excecute.
I've found most slow progressing students are not actually deficient in playing ability but more learning ability. I try to tell them not to mix practicing and playing. Each should be taken seperately. Because if you mix the two is hard to balance it, usually playing takes over from practice. Thats what i have found works anyway. Most students makes constant progress if they clearly understand the two different piano playing.
For students who struggle to play a bar I simply draw a line in the middle of the bar, or close to, seperating the groups of notes. And then another line seperating the two staves. Just work in the four quadrants, top left is 1, bottom left is 2, Both hands together, that is 1 and 2 will be step 3. The top right quadrant is 4, bottom right is 5, those two together is 6. Then both 3 and 6 together is the last step 7. Dont move onto the next until the previous one is played without any note errors or fingering mistakes. I give this to little 5 year olds or older students who wont tell me their age lol, they all seem to make progress tackling the bars.
Still its pretty logical for most of us to just learn RH and then LH then put it together, but i have found for begnining students or students with struggle, seeing their achievment in seven steps is fast rewarding and an obvious indicator to them, when their teacher isnt around, to they know their work or not.