On a very long page, we were not able to select the fields towards the bottom of this page. We found the following in PeopleBooks
Home -> PeopleSoft PeopleTools 8.53 -> PeopleSoft Application Designer Developer’s Guide -> Understanding Page Controls -> Data Entry Controls
Note: PeopleSoft Application Designer allows you add up to 405 fields on a page. When you reach this limit, typical system behavior includes being unable to select, move, or edit the page fields with an order number exceeding the 405-field limit. While this limit has no effect on the page at runtime, it has the potential to disrupt productivity during design time. If you find a page nearing the 405-field limit, you should use subpages to decrease the number of fields on a single page.
Below PeopleSoft resolution describes the same issue which is there since Version 8.2
E-AD Is there any limit on the number of fields on a Page Definition ( Doc ID 648823.1 )
Using Sub pages is the easiest way to resolve this issue. However this require changing the code.
The below query can be used to identify any such pages
select pnlname,count(*) from pspnlfield group by pnlname order by count(*) desc