I used to run across this problem when using it to wash laundry in the washer in place of washing powder; turns out the washing soda I had was only 30% sodium carbonate with no mention of the main chemical(s), "trade secret" I guess. It consisted of almost translucent crystals and refused to dissolve. Is that what you have?I have bought lime and washing soda, but have run into a technique problem. When I mix the washing soda with water, it absorbs the water quickly and turns into a block of concrete in the bottom of my jar.
I ditched that one and purchased another that says something to the effect of "100% washing soda". This one is a white powder, dissolves without issues and does give off some heat as described by Barefootboy. So far so good.
Getting my hands on hydrated lime was a lot more difficult. Salespeople looked at me like I was crazy; it was too cheap to bother ordering one of these. In desperation, I tried to make the lye using lime putty instead of hydrated lime... that gave me a solution of about 5% of lye (enough to remove paint off of a wooden door, but not enough to make soap with). I am ready to revisit that option, however, since now that I was able to finally get my hands on genuine, hydrated lime from a mined source (turns out they can manufacture this stuff using drain-cleaner quality lye, the very thing I am trying to avoid), my "soap" making produced something akin to soap flakes in immersed in sunflower oil (see picture).
Sunflower oil may not be the most suitable oil for a soap experiment, but that's all I had... Any ideas?