I just finished re-reading This Present Darkness, by Frank Peretti. He has angels prompting people to pray when the spiritual battle becomes hot and heavy. It reminded me of times in the past when God's people have been wakened in the night to pray for someone far away who is in need of prayer at that moment. It happened frequently with saints who've gone before us.
I believe it's the Holy Spirit, not angels, who does the prompting, but it seems like a long time since I've felt that nudge. It bothered me, and I asked the Lord why.
He said, "How long has it been since you asked how to pray?"