If you are switching between coins of the same algo then you will only see the hashrate of the current coin being mined, MPH does not remember what your past hashrate was.
Some coins can take a long time to display the current hashrate and if the miner changes (AFAIK) before the hashrate is displayed you may never see the hashrate.
If you are trying to compare MPH reported hashrate against your miners reported hashrate probably the best thing to do would be to mine each coin individually for a while (24 hours each) and compare the results.