The problem area appears to be the fingerprint scanner or its supplier
As surprising as it might sound but it is the truth (if the report has to be believed) that Samsung came across a bunch of hurdles while putting together the Samsung Galaxy S5. The problem area was the most probably the fingerprint scanner or maybe it was just the trouble with the supplier of it but in both cases it had to something with it. We can easily imagine Samsung having trouble with its fingerprint scanner because an announcement was made last year to have one on the Note 3 but that never really got pulled off by Samsung because apparently they were not able to get satisfactory results from the suppliers.
Following this incident, when Samsung had announced the fingerprint scanner as being part of the upcoming Galaxy S5, it decided to produce its own fingerprint scanners to avoid having any troubles with external suppliers. Unfortunately, this couldn’t happen and the company once again had to resort to the suppliers (Crucialtec in this case). The companies are expected to enter into a contract soon enough and well before the phone comes out for sale in April.
source: hi-tech.mail (translated)