The click commits records and this is needed to get the summary field to update. It's normal but irritating behavior for a summary field used in this context. You can sometimes script the commit records from a script trigger on your field(s) where an edit will change the total shown or you can use a calculation field defined in the parent record that uses an aggregate function such as count or sum to compute and display the same value. In general the aggregate functions used in this context will update more smoothly than a summary field defined in the related table.
Thanks PhilModJunk I've replaced the Summary field (based on the child table) with a calculation field within the parent table and it works fine.!