Boiling point of ethanamide vs propanamide
The relatively high boiling points for such low molecular weight compounds suggests a primary effect from hydrogen bonding.

Using ethanamide as a reference point, propanamide has a higher molecular weight, which should raise the boiling point a little, but three relatively nonpolar carbons (plus hydrogens) vs two in ethanamide. A lower boiling point would then not be unexpected. Butanamide seems to support this, being a little higher in bp than propanamide by virtue of its higher mw, but still lower than ethanamide because of its lower overall polarity.

But to throw in some difficulty, formamide (methanamide) has a bp of only 210 °C, whereas its low content of nonpolar atoms suggest it should be much higher.

And then there are the melting points, which are difficult to relate to polarity: formamide 2 °C; ethanamide (acetamide) 80 °C; propanamide (propionamide) 80 °C; and butanamide (butyramide) 115 °C.

Formamide could be an outlier because of nitrogen flipping back and forth thru almost-planar nitrogen. Butanamide's high mp is a bit surprising. There could be packing factors in the solid that are favorable.

Maybe too many factors and too small overall effects to be able to make a lot of predictions, even tho we may be satisfied with some explanations after seeing the numbers.