If you're using the 7th edition of Wheelock's Latin (the most recent edition), this is explained at the bottom of page 4:
Note that the stem vowel has no macron in certain forms (e.g., moneō, laudānt); learn the following rule, which will make it easier to account for macrons that seem to disappear and reappear arbitrarily:
Vowels that are normally long are regularly shortened when they occur immediately before another vowel (hence moneō instead of *monēō), before -m, -r, or -t at the end of a word (hence laudat, not *laudāt), or before nt or nd in any position (laudant, not *laudānt: the asterisks here and elsewhere in this book indicate a hypothetical form not actually occurring in classical Latin).
The 7th edition was significantly revised/expanded; I don't remember whether earlier editions have a similar explanation.