How do I reach all students with sight word instruction?

So, I have been collecting data for a few weeks now. The students have been introduced to 10 sight words and have been doing lots of reading, writing, and practicing every day in small group. Since I have been absent a couple of days and we have a field trip last week that took away from our groups, I did a quick assessment of all ten words on Friday. I have been using constant time delay with the students each day we meet, which continually goes back and reviews words they do not know. However, I wanted to mix up all 10 words and have the students read them without using the constant time delay technique. Noah was absent again so was not tested. I found that Nicole new 9 words, only missing "of". Ellen knew 9 words, only missing "from". Ted knew 8 words, missing "this" and "from". Ben and Jon were a slightly different story. Surprisingly, Ben missed 5 words, so he only read 5 words correctly. Jon missed 7 words, only reading 3 words correctly. Ben has been doing well in the group. He is, however, very inconsistent and misses different words every time. Jon, is who I am mostly worried about. He struggles in group each day to read the sight words during the constant time delay, reading the sight word readers, and sight word activities. He is the student that requires the most reminders to stay on task and focus. I believe his lack of focus has to do with his difficulty and frustration in reading sight words. Based on his parent survey he is practicing sight words every night during the school week. If he is practicing at home and at school each day, why is he having so much difficulty reading sight words? We have recently started using a sight word "gum ball" machine to help students track their progress with sight words. When they read all their weekly sight words correctly then they get to put a sticker on the gum in their "gum ball" machine. When all their pieces of gum have stickers they earn a real piece of gum from the gum jar. The students seem very motivated by this and are very excited each day to read their words. Below is a picture of Jon's "gum ball" machine:

Here you can see he has been able to read 6 sight words. He read these words during the morning before we started our day. Then, during our small group at 10:30 he missed several of these words, including "to" and "and". I notice that he is engaged when reading the "gum ball" machine. Maybe in small group he is unable to focus because he is not interested in the activities I am doing. I am going to include their "gum ball" machines in our lesson tomorrow and see what happens to his attention then.