Why do we have parallel sessions block within the same sql query

we have been experiencing a trend to get some sql statements running long and, apparently, blocking because a parallel thread blocks other threads for that session (reported as: Above sql just finished and moved to next step but looks like one of the session in parallel threads is blocking the other session for same query.)

typically, not every table referenced in the query has a degree of parallel set and also they are typically not including the parallel hint.

has anybody run into this previously and is there a recommended way to prevent this and improving performance (I hesitate to suggest they use a noparallel hint to avoid this)