I have the same problem you describe. I attribute this to the dog teeth not being aligned with the gears at start-up, while the geartrain is not moving. Of course, in your case this also happens when moving between 1st and Reverse, but I'll venture to guess the mechanism of the problem is similar.
My solution is to push the lever into a higher gear, say 4th, and this typically "bumps" the geartrain enough that I can engage 1st or Reverse. Piece o' cake.
HTH!
