As a teacher there is always a certain amount of work you need to get through in an academic year. This can often leave very little time to fit in other things and with pupil absences, school trips and holidays you are left with little movement. Often children of a young age forget things quite quickly so it is important that you revisit things time and time again to allow them to understand and memorise it. It may be that most of the pupils have understood something but a small group are still struggling. If this is the case then you may need to work with them a little more closely or ask your TA to do some extra work with them when you are doing another task that they could miss out on.
You may not have to go over the topic in the same way. Often teaching them the same principle but in a different way can help them to really get to grips with it. If you are struggling for ideas then you could search online or ask other teachers how they went about it.
Another way to give pupils a bit more time on a task is to set them some homework that is themed around the subject you have been teaching them about.