chocolate fudge not having harder or sticky consistency
If condensed milk and chocolate chips are, basically, your only ingredients, make sure you are using sweetened condensed milk and not just evaporated milk.  The 5-minute fudge recipe has been a staple of bake sales and church socials for as long as I can remember and should work on a 3-minute microwave cycle. Some recipes mix the types of chips (milk chocolate + semi-sweet chocolate) - some recipes throw in a couple tablespoons of butter into the mix (Eagle Brand's recipe is more purist, though). But sweetened condensed milk and a LOT of chocolate chips should do the trick.

http://www.eaglebrand.com/recipes/details/?RecipeId=3968

The only other problem I can think of would be if you microwaved your chips alone first and they got so hot that the chocolate seized - or if you used some kind of chocolate flavored chips that were made from mystery materials.

I don't like to use the microwave to melt chocolate - ever.  For a recipe like this, I would melt the chocolate into the sweetened condensed milk over a double boiler or in a heavy saucepan over low heat - sometimes slow and steady wins the race.