You should certainly make sure your starter is freshly fed and grown and not proteolytically liquified from storage before using it. But a flat loaf is more likely the result of other factors during fermentation that weakened the gluten: poor gluten development (not enough appropriately gentle folding, etc) or more likely, over-fermentation. Try to err on the side of underfermentation next time and see if it holds its shape better. Crumb may not be as open, but loaf should stand taller. It's a delicate balance and takes experience to develop the right instincts.
You should certainly make sure your starter is freshly fed and grown and not proteolytically liquified from storage before using it. But a flat loaf is more likely the result of other factors during fermentation that weakened the gluten: poor gluten development (not enough appropriately gentle folding, etc) or more likely, over-fermentation. Try to err on the side of underfermentation next time and see if it holds its shape better. Crumb may not be as open, but loaf should stand taller. It's a delicate balance and takes experience to develop the right instincts.
Tom