There are two cases when this happens:
1. You have opened a deal on the pair with zero swap. Below is a list of such pairs: http://instaforex.com/en/specifications
2. SWAPs have been charged, but the amount of charged swaps is less than the minimum possible amount displayed in the platform.
Please note that funds in the account are displayed to within $0.01. For example: the swap rate for the GBP/USD pair is 0.20 pips, i.e. for 1 lot, it will be 0.20 USD (1 USD * 0.20). However, for a deal on the same currency pair with the trade size of 0.01 lot, the amount of the accrued swap will be equal to 0.002 USD (Pip value of 0.01 USD * 0.2). This amount cannot be displayed in the platform. If you have kept your position open for five days, the swap will be displayed in the platform as follows: 0.002 * 5 = 0.01 USD.