Packet Dropped seen from `ifconfig` could be due to many reasons, you should dig deeper into NIC statistics to figure out real reason. Below are some general reasons

 - NIC ring buffers getting full and unable to cope-up with incoming bursts of traffic
 - CPU receiving NIC interrupts is very busy and unable to process 
 - some cable/hardware/duplex issues
 - some bug in NIC driver

Look at the output of

* `ethtool -S wlan0`
* `iwconfig wlan0`

and the content of `/proc/net/wireless` for any further information.