Multiple Regression

I am trying to understand the relationship between livestockunits(X) and gross margin earnings(Y). I have generated different models using multi variables inaddition to my main independent variable.
In one of the multi Regression model, individual t-test of the main independent variable is not significant at 5%. But overall F-test of all variables is significant at 5%. Is this due to multicollinearity issue and Omitted Variable Bias? And R squared only explains 2% of the variation of Y. With my limited understanding, i have planned to add omitted variables and try to improve my variation? Is this correct way of doing it?