If you have heavy clay soil you need to be especially careful about amending planting holes. You run the risk of the potting soil/amended soil retaining too much moisture that does not drain well through the native clay and you wind up with a 'bucket effect' of soggy, poorly draining soil that can be the kiss of death for most plants.

Ideally you want to amend as large an area as possible. Rather than amending individual planting holes, amend the entire planting bed. The intent is to make the soil texture as uniform as possible over the largest area and ensure that drainage is relatively prompt.

Often with heavy clay or poorly draining soils that is not immediately practical give the size of the area in question or existing established plantings that preclude widespread amending. In that case, it is better to just dig the largest but rather shallow planting hole you can manage, loosening up the existing soil as well as possible. Add no amendments to the hole - existing soil coming out of the hole is the same soil used to refill. The shallow, widely dished planting hole will encourage you to plant high......just mound soil upto to the necessary level, creating a small mound or berm. This will facilitate good drainage yet still accomplish the recommended deep planting routine for clems. If you have insufficient garden soil to make a deep enough mound to cover the rootball and stems, this is where you can augment with some sort of organic amendment, like compost or potting soil/garden mix, etc.

It is my experience that clematis - like many plants - are rather forgiving to their growing conditions. Obviously it is prudent to give them the best start possible but if that has not been case for whatever reason, it it still possible to grow great clematis vines once you understand their specific needs and address them going forward. Unfortunately poor drainage conditions can be extremely harmful and rather pernicious in their effect on plants - if the root system has already been compromised by poor drainage, there's not a lot more you can do, other than to try again!

I have heavy clay soil as well. I typicalling dig a large whole, discard the blue clay and add Home Depot's compost mix (cost about $1.40 per bag) and mix it in with the original soil. I have had a good success rate. I would never remove all the original soil and replace potting soil, because of the "pond effect".