Hi,
This sounds like a practicing issue. Do you ask your students to record how much time they are practicing? I do, mostly because it helps them to reflect if they are practicing daily or not. I tell my students that 5 minutes every day is better than 2 hours on one day. For some students, especially the ones taking advanced classes in school, 30 minutes of practice is getting in the way of their homework time. If they take 5 - 10 minutes between subjects or as a break during homework, it is easier for them to manage. Doing this several times in one day will quickly add up to 30 minutes or even an hour every day.
I also ask my students to set a goal for each practice time. Practice these two measures until you can play it 3 times in a row perfectly. Then we will do that during the lesson. After they play a passage I'll ask, "How was your rhythm?" or "Did you play all the right notes?" This helps them learn to critique their own practice time at home and to recognize that practice is not just playing, but playing for a specific end in sight. By practicing during the lesson, I'm teaching them what to do at home.
Sometimes during a lesson, after the student has played their piece, I'll ask them if they were me, what would they say? A lot of times the students already know how their piece needs to improve. Then I'll ask them how they plan to fix that issue. Sometimes they give me great ideas, sometimes they need help in knowing how to approach the issue.
With rhythm, I have found 2 techniques that work pretty well. One is that I will cut pieces of paper to match the rhythm note. For example, a quarter note will be 1 inch long and a half note will be 2 inches long. Then I'll cut a dotted quarter note, 1 1/2 inches and ask how long do you hold this? The lengths of paper seem to give the students a visual picture that they can understand. Another technique is that I'll ask the students to write where beat one starts, where beat 2 starts, etc. then we talk about the notes in between and what to say on those notes.