diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -5,7 +5,7 @@ import multiprocessing
 import logging as log
 
 from hashtree import hash_block
-from netnode import BaseConnection,NetNode
+from netnode import BaseConnection, NetNode
 import config as conf
 import status
 from datafile import DataFile
@@ -150,8 +150,10 @@ class Server(NetNode):
 				self._new_leaves[k+self._tree.leaf_start] = hash_block(block)
 
 		t = datetime.now().timestamp()
-		if t-self._last_flushed>=60 and self._tree_file:
-			self._update_tree()
+		if t-self._last_flushed>=60:
+			if self._tree_file:
+				self._update_tree()
+			self._refresh_lock()
 			self._last_flushed = t
 
 		return ({"command": "ack", "index": indices},)