not all statements are sending

We've been having ongoing issues for quite a few months now whereby the full list of selected statements to be emailed are not sending. 

I used to be able to 'select all' & the full list of 200 or so statements would all send sucessfully.  No problems. 

Now even if I only select a batch of 40, *maybe* three of them will appear in the sent list.  The rest do not appear in the sent list at all.  And it is very hit and miss.  Very occasionally I'll optomistically 'select all' and they will all send...'Great!, It's working', I think...except then it's straight back to NOT sending them the next time.  

If I go in and select 5 at a time they will *usually* all send (although even then, sometimes only 3 will go)...but this is tedious, inneficient and should not be necessary. 

I have read other threads of people having the exact same issue, seemingly with no resolution.  I have read the previous suggested causes & none of them seem to apply.  

It is incredibly frustrating that nothing appears to have been done to resolve this problem.