The specifications for science exams are big and some things will get more focus than others. Some aspects never get taught very much at all, and some are not taught well consistently every year. Here are some reasons why...
Changes to the specification: When a science specification changes, only about 5% is different. In a way, this makes it harder to write new resources than a 100% change, because the resources are almost sufficient, but not quite and it's easier to spot a small change in the specification than to just start with a blank slate. This problem should lessen with time as teachers become more familiar with the specification.
Also, when AQA released its 2016 specification, it posted a summary of changes. The page has been taken down since then (probably because of the passage of time making the document less relevant), but it also releases some news articles of changes if they happen, such as this one.
Some parts of the specification are considered easier than others: Some parts may be considered easier if they don't contain difficult concepts that require a lot of practise or mathematical skills. The most obvious one from AQA A level chemistry is the transition metals unit at the end of year 13 - it is a list of transition metal reactions and transition metal colours that students need to learn.
These perceived easier topics are given less class time and students may be asked to learn them themselves. In my experience as a tutor, however, the students are not learning them very well themselves for a few reasons as the topics are probably not 100% memory. In the example of transition metals, you have to explain using entropy why certain reactions happen, or why 3+ ions are more acidic than 2+ ions.
The parts of the specification are taught at a "dip" in the year or not given enough time:
The transition metals topic mentioned in the last point is also one of the last topics taught in A level chemistry. If the students are running out of time, it might be taught in a rush. Another topic that may not be given enough time is NMR spectroscopy - it requires a lot of practise to interpret an NMR spectrum correctly and it is normally taught after most of the organic chemistry.
The end of year 12 is also a great time for whatever gets taught to get forgotten - it's after the year 12 exams and students are generally tired.
One way around it is to use the one advantage terminal exams provide - since there are no year 12 AS levels to revise for, you can teach/learn the specification in whatever order you like. You could use the transition metal examples when first teaching entropy or acid strength. You can also start NMR earlier to give students more practise. Look at the topics that are difficult and teach them earlier - they all get examined at the end of year 13, so there is no reason to follow the order of the specification.