If you've got X staff doing the work of X+y staff you end up with problems.
It can be that simple, and given the police have had their funding slashed and lost a massive number of the backroom staff who were originally employed to deal with specialist parts of the paperwork you are bound to get an increase in problems with the paperwork, especially in serious and complicated cases where there will be intense scrutiny of every step of the investigation.
Things that would be picked up by someone who deals with it daily might not be noticed by someone who deals with it only a few times a year (exactly the reason you have lawyers who specialise not just in Criminal or Civil law, but often very specific parts of it IE driving offences, sexual offences etc).
One of the biggest lies the Conservatives (especially May as HS) told the country in regards to "Austerity" was that cutting the police funding so they let go of backroom staff wouldn't affect front line policing, it was an load of obvious BS, as you inevitably ended up with police officers doing work that had previously been done by specialist support staff who were both cheaper and often better at it (not because the police aren't good at their jobs, but because a lot of those backend jobs were specialist, a bit like expecting a general car mechanic to know how do fix an ECU or deal with the hydraulic suspension on a specific model of car that was the only one to use that system)