Robbing is not unheard of but a little unusual in spring when there’s a flow on - all the bees are too busy and satisfied with the abundant nectar to try getting it the hard way. Could it be that your colony has just had a bunch of new workers emerge and doing orientations?
If you’re reasonably sure it’s robbers, you can buy or make a robbing screen, and I always have good luck with draping a damp beach towel over the front of the hive for 1-2 days: