Why couldnt the lannisters raise a new army during the dance like Tywin tried to do with stefford lannister?

Good question!

And ultimately this is why I have a problem with the army sizes during the Dance, which in turn are part of my overall problem with the Dance as military history. Where we have numbers to tell, the armies of the Dance are pretty small by the standards of later Westerosi wars:

• Battle of Rook’s Rest: >800 greens, 100 blacks.
• Battle of the Gullet: ~100 ships on both sides.
• The Fishfeed: At least 2000 greens, at least 3,100 blacks.
• The Butcher’s Ball: 3,600 greens, ~7,000 blacks.
• First Tumbleton: >9,000 greens, ~7,000 blacks.
• Second Tumbleton: <= 9,000 greens, 4,000 blacks.

While one could argue that, post-Aegon’s Conquest, the Westerosi had shifted to a model of having multiple smaller armies rather than one big host to avoid losing everything to one dragon, this creates another problem. 

We know from later wars that the various regions of Westeros can field much larger armies in the several tens of thousands, so if that is the case, the various regions of Westeros should have had more armies in the field at one time, and should been able to raise new armies and be ready to keep fighting. 

Moreover, these numbers create new problems for historical consistency: if only 2,000 or so Westerlanders marched east with Jason Lannister, then the Westerlands couldn’t have been “thinly defended,” and so Dalton Greyjoy’s reaving should have been met by 43,000 Westermen ready to defend their homes against the 15,000 Ironborn. But since we know the Westerlands were “thinly defended,” then the casualties at the Red Fork and the Fishfeed should have been larger by at least an order of magnitude.