I followed the instructions but couldn't get the HS210 3-way switch to work.  It does turn the light on but turns it off right away and it keeps doing that every 2 or 3 seconds.

Here's the 1st switch, where I installed HS210.

Here's the 2nd switch, which I left unchanged as it doesn't appear to have a neutral wire.

Appreciate it very much if anyone can shed some light on this!