The following applies if you are using a balance forward statement presentation.
When you close the statement cycle, it rounds up all new documents that have been on a prior statement and have a balance and totals them to be the balance forward amount. It rounds up all new documents that have not been on a statement and prepares them for detail on the statement and marks them as belonging to the statement run dated when you closed the statement cycle. So, if these items were part of a close statement run done prior to your latest run and the latest run was on a date prior to the most recent closed date, those items are now part of the beginning balance and do not show as detail.
The most common thing that happens is that the user either accidentally closes the statement cycle prematurely or closes it, reviews the statements and finds that they forgot to enter some charges so they enter those charges and close the statement cycle again but now on the same day the originally closed the statement cycle thereby causing the situation I believe you are describing. There is a process you can use to reclose the same statement cycle of subsequent days (closing more than once on the same day is fine) but, once you don't follow that, you will have the issue.
There are a couple of options to re-expose these documents as detail but before going into that we should confirm this is the case. You can open the close statement cycle screen and look at the last closed date and see if that date is beyond when you first closed the statement cycle. You can also run a SQL query similar to the following to further explore this.
select * from ardoc where docbal <> 0
Look at the field called StmtDate on the documents that are not showing but you think they should be and at the date of some documents that are showing. If the dates are different, you have this issue.