If one did a hefsek tahara and it wasn’t good, can she try again?
A woman is allowed to try several times to do a hefsek tahara, until it comes out clean.
Sometimes, the bleeding has stopped, but there are small amounts of blood that remain inside. Therefore, it is possible that when she does a bedika, the bleeding has really stopped and she is just finding this blood that is left over. Therefore, sometimes the second or third bedika might come out better, since the first one cleaned out the area.
It is important to note that the hefsek tahara must be done before shkia (halachic sunset, see myzmanim.com for times in your area). If she does multiple bedikot, the last one must be completed before shkia for it to be valid (if it was indeed after shkia, a Rabbi should be consulted).
In between each try, she should wait 10-15 minutes. In a time of need, she may wait as little as 3 minutes between each try. If she waited less than that, a Rabbi should be consulted if the hefsek tahara is valid.
It is recommended not to try more than 3 times, since too many bedikot in such a short time can cause sratching and irritation. Also, if it didn’t come out clean in the first 3 times, it probably won’t get any better even if she tries again.