Uploading Big sketches to Arduino Uno

i am banging my head off a problem and wondered if anyone else has had similar.
i’m writing a sketch which uses two large data files, one 16K and one 1K ( PROGMEM char ChCL128 = { 16K data}; ) and
( PROGMEM char ChC08 = { 1K data}; )
When i try to upload to Arduino i get the error message Avrdude verification error, first mismatch at byte 0x0580 0xff != 0x46
if i take the 16K data file out of the sketch (leaving just the 1K data) it uploads ok. it looks as though uploading the 16K might be going into an area it shouldn’t in Arduino.
the total size to upload is 24,650 bytes which should in theory be ok as Ard has 32k free
any ideas would be appreciated