I have suffered from this problem many a time. Sometimes the solution is actually more meth. I find I’m super excitable after another big hit when I’ve been limp for a while. I can usually get an hour out of that. I do often use viagra ( noting the risks of using it with meth) and find 50 mg will do the trick but only if taken after the bulk of my meth is already been consumed. It works less well when taken in advance. The third trick, useful when being orally pleasured, is to turn your mind to your hottest fantasy and stop thinking about what your partner of the moment is doing. Finally, there’s no beating a sensibly sized toy lovingly placed in your ass and manipulated in tempo with your blowjob - that not only fixes the limp ness but will usually bring on the very hard to achieve meth orgasm.
Note: all these things are highly effective when combined....