ZLib won't compress beyond ~600kb output in D - d

Question

I've tried both std.zlib and using zlib directly, but it doesn't seem to compress anything beyond the 600kb mark. I have mostly followed all online guides except for the input buffer since I have to compress a preesixting memory space rather than a file on the disk.
Forgot the code, this one doesn't go further beyond ~600kb:
int ret, flush;
zlib.z_stream strm;
strm.zalloc = null;
strm.zfree = null;
strm.opaque = null;
ret = zlib.deflateInit(&strm, compLevel);
if (ret != zlib.Z_OK)
throw new Exception("Compressor initialization error");
ubyte[] output;
output.length = cast(uint)imageData.length;
strm.next_in = imageData.ptr;
strm.avail_in = cast(uint)imageData.length;
strm.next_out = output.ptr;
strm.avail_out = cast(uint)output.length;
do{
ret = zlib.deflate(&strm, zlib.Z_FINISH);
if(!(ret == zlib.Z_OK || ret == zlib.Z_STREAM_END)){
//version(unittest) std.stdio.writeln(strm.total_out);
zlib.deflateEnd(&strm);
throw new Exception("Compressor output error: " ~ cast(string)std.string.fromStringz(strm.msg));
}
} while (ret != zlib.Z_STREAM_END);
writeBuffer = cast(void[])[Chunk(cast(uint)strm.total_out, DATA_INIT).nativeToBigEndian] ~ output[0..cast(size_t)strm.total_out];
file.rawWrite(writeBuffer);
crc = crc32Of(writeBuffer[4..$]).dup.reverse;
file.rawWrite(crc);
Previously I tried this, but this immediately has a stream error with no output:
int ret, flush;
//uint have;
zlib.z_stream strm;
strm.zalloc = null;
strm.zfree = null;
strm.opaque = null;
ret = zlib.deflateInit(&strm, compLevel);
if (ret != zlib.Z_OK)
throw new Exception("Compressor initialization error");
ubyte[] output;
static if(writeblocksize < 2048)
output.length = 2048;
strm.next_in = imageData.ptr;
strm.avail_in = cast(uint)imageData.length;
do {
flush = strm.avail_in ? zlib.Z_NO_FLUSH : zlib.Z_FINISH;
strm.next_out = output.ptr;
strm.avail_out = cast(uint)output.length;
ret = zlib.deflate(&strm, flush);
if(ret == zlib.Z_STREAM_ERROR){
version(unittest) std.stdio.writeln(ret);
zlib.deflateEnd(&strm);
throw new Exception("Compressor output error: " ~ cast(string)std.string.fromStringz(strm.msg));
}
//version(unittest) std.stdio.writeln(strm.total_out);
//writeBuffer = output[0..$-strm.avail_out];
writeBuffer = cast(void[])[Chunk(cast(uint)writeBuffer.length, DATA_INIT).nativeToBigEndian] ~ output;
file.rawWrite(writeBuffer);
crc = crc32Of(writeBuffer[4..$]).dup.reverse;
file.rawWrite(crc);
//writeBuffer.length = 0;
} while (flush != zlib.Z_FINISH);
zlib.deflateEnd(&strm);