This is 1 year too late, and my apologies for starting this with a question.
Do you happen to work on geographic coordinates (latitude-longitude) such as WGS84 ?
[This answer is assuming you are on Lat-Lon environment. Otherwise please wave this off...]
I suppose Spline Tool is a great tool, but it was probably designed for projected Xm-Ym coordinate system.
It monitors X-Y distance between our mouse-clicks, and automatically calculate and update best fitting spline curves.
It should work great if we move our mouse 50m to the next point, but it may mean somewhere around 0.0005 degrees (Lat-Lon-wise). Spline Tool cannot tell what happened.
To signal Spline Tool that our mouse has moved, we need to set the Tolerance to very small number, say 0.001 or even 0.000001, depending on the scale we are working on.
Unfortunately its user interface is not intuitive enough, as the Up/Down arrows changes only +1.0 or -1.0 incremental value...
You will see how Tolerance affects your result in the image below.
In this test, I tried to draw a polygonal line of 4 points on a map view at 1:25,000 scale (whatever it means) as shown in (1); starting from lower left, goeing up, then down, and backing up again to stop at upper right corner.
Then I activated Spline Tool, and applied various Tolerance values:
- (2) Tolerance: 1.0
- (3) Tolerance: 0.001
- (4) Tolerance: 0.0001
- (5) Tolerance: 0.00001
- (6) Tolerance: 0.000001
You will see neither (2) 1.0 or (3) 0.001 do not show any effect at this scale.
But it started to work at (4) 0.0001, and (5) 0.00001 looks fine to me. However (6) 0.000001 seems rather too busy.
Anyway you should be able to find best Tolerance value, fit for your task.
This test was made using QGIS 2.14.7 on Windows, but probably applicable to other versions (2.0 and after).