Your budget is calculated each period based on your company's financial success. If you make more money, you'll get a larger budget. If you lose money, your budget will be smaller.

Unused budget does not carry over to the next period. If you don't use all of your marketing budget, the amount you didn't use will no longer be available.  The budget is recalculated each period based on the performance of the firm.

This could be from one of 2 causes:

1. The marketing budget is primarily a function of sales and income and is recalculated each period. If your sales and/or income dropped from the previous period, your budget will be adjusted accordingly.

2. You just introduced a new product last period, at which point your budget would have been increased by a fixed amount. This allows you to launch your new product, as it has no previous performance to calculate the budget from. For the period after the launch (first period of sales of new product), the budget will be recalculated based on your actual performance. This recalculation may result in a reduction in your budget.