diff --git a/bulk_indexer.go b/bulk_indexer.go
index 5fcf50c..07e9745 100644
--- a/bulk_indexer.go
+++ b/bulk_indexer.go
@@ -232,9 +232,10 @@ func (b *bulkIndexer) Flush(ctx context.Context) (BulkIndexerResponseStat, error
 	if b.maxDocumentRetry > 0 {
 		if cap(b.copyBuf) < b.buf.Len() {
 			b.copyBuf = slices.Grow(b.copyBuf, b.buf.Len()-cap(b.copyBuf))
-			b.copyBuf = b.copyBuf[:cap(b.copyBuf)]
+			b.copyBuf = b.copyBuf[:b.buf.Len()]
 		}
-		copy(b.copyBuf, b.buf.Bytes())
+		n := copy(b.copyBuf, b.buf.Bytes())
+		b.copyBuf = b.copyBuf[:n]
 	}
 
 	req := esapi.BulkRequest{
@@ -275,7 +276,7 @@ func (b *bulkIndexer) Flush(ctx context.Context) (BulkIndexerResponseStat, error
 
 	// Only run the retry logic if document retries are enabled
 	if b.maxDocumentRetry > 0 {
-		buf := make([]byte, 0, 1024)
+		buf := make([]byte, 0, 4096)
 
 		// Eliminate previous retry counts that aren't present in the bulk
 		// request response.
@@ -307,8 +308,8 @@ func (b *bulkIndexer) Flush(ctx context.Context) (BulkIndexerResponseStat, error
 		}
 
 		// keep track of the previous newlines
-		// the buffer is being read lazily
-		seen := 0
+		newlinesInBuf := 0 // number of newlines currently in `buf`
+		seen := 0          // number of newlines previously read (excluding the ones in `buf`)
 
 		for _, res := range resp.FailedDocs {
 			if res.Status == http.StatusTooManyRequests {
@@ -336,43 +337,53 @@ func (b *bulkIndexer) Flush(ctx context.Context) (BulkIndexerResponseStat, error
 				b.retryCounts[b.itemsAdded] = count
 
 				if b.gzipw != nil {
-					// First loop, read from the gzip reader
-					if len(buf) == 0 {
-						n, _ := gr.Read(buf[:cap(buf)])
-						buf = buf[:n]
-					}
-
-					newlines := bytes.Count(buf, []byte{'\n'})
-
 					// loop until we've seen the start newline
-					for seen+newlines < startln {
-						seen += newlines
-						n, _ := gr.Read(buf[:cap(buf)])
+					for seen+newlinesInBuf < startln || len(buf) == 0 {
+						seen += newlinesInBuf
+						n, err := gr.Read(buf[:cap(buf)])
+						if err != nil && err != io.EOF {
+							return resp, fmt.Errorf("error reading gzip data: %w", err)
+						}
 						buf = buf[:n]
-						newlines = bytes.Count(buf, []byte{'\n'})
+						newlinesInBuf = bytes.Count(buf, []byte{'\n'})
+						if err == io.EOF && seen+newlinesInBuf < startln {
+							return resp, fmt.Errorf("could not find first newline: %w", err)
+						}
 					}
 
 					startIdx := indexnth(buf, startln-seen, '\n') + 1
 					endIdx := indexnth(buf, endln-seen, '\n') + 1
 
-					// If the end newline is not in the buffer read more data
-					if endIdx == 0 {
-						// loop until we've seen the end newline
-						for seen+newlines < endln {
-							// Add more capacity (let append pick how much).
-							buf = append(buf, 0)[:len(buf)]
+					if endIdx != 0 {
+						// The whole document is in buf - write it and move to the next one.
+						b.writer.Write(buf[startIdx:endIdx])
+						continue
+					}
 
-							n, _ := gr.Read(buf[len(buf):cap(buf)])
-							buf = buf[:len(buf)+n]
+					// We only have the beginning of the document. Write what we have so far.
+					b.writer.Write(buf[startIdx:])
 
-							newlines = bytes.Count(buf, []byte{'\n'})
+					// Loop until we've seen the end newline.
+					for seen+newlinesInBuf < endln {
+						seen += newlinesInBuf
+						n, err := gr.Read(buf[:cap(buf)])
+						if err != nil && err != io.EOF {
+							return resp, fmt.Errorf("error reading gzip data: %w", err)
+						}
+						buf = buf[:n]
+						newlinesInBuf = bytes.Count(buf, []byte{'\n'})
+						if seen+newlinesInBuf < endln {
+							if err == io.EOF {
+								return resp, fmt.Errorf("unable to find end newline in gzip data: %w", err)
+							}
+							// We haven't found the end newline yet - write the whole buffer.
+							b.writer.Write(buf)
 						}
-
-						// try again to find the end newline
-						endIdx = indexnth(buf, endln-seen, '\n') + 1
 					}
 
-					b.writer.Write(buf[startIdx:endIdx])
+					// End newline found
+					endIdx = indexnth(buf, endln-seen, '\n') + 1
+					b.writer.Write(buf[:endIdx])
 				} else {
 					startIdx := indexnth(b.copyBuf[lastIdx:], startln-lastln, '\n') + 1
 					endIdx := indexnth(b.copyBuf[lastIdx:], endln-lastln, '\n') + 1