When a steam locomotive was built (most of them anyway), it was designed with an adhesion factor of 4 (more or less). This meant that the theoretical tractive effort was 25% (4x) of the weight on the drivers. For tank engines, this would have to include a situation where the engine was low on fuel/water (and resulting weight) in the tanks over the drivers.
So naturally, a fully supplied tank engine would have more weight on the drivers, a higher factor of adhesion and be less likely to slip.
Converting an engine from tank to tender will always make it SEEM to be splippery compared to what it was, because it no longer has the extra weight on drivers. Extra weight (iron) was sometimes added to compensate for the loss of the weight of the tank itself and to maintain a factor of 4 or 25%. On some converted engines (especially articulateds), I would suspect the failure to rebalance the engine on the springs might have been a contributing factor also.
Diesels also were designed for a nominal 25%, but modern computerized wheel slip systems and AC drive have pushed into 40%+.