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No doubt, that shutter (in a device this old) is sticking.
You may need to disassemble the projector enough to assess the situation, possibly lubricate the shutter a bit (if cleaning it and the surrounding mechanism/housing does not free up its function), or other things.
Would you be up to doing that work yourself? or would you prefer another person do the work, whether a friend or someone you would hire to do the work?
Or would you prefer not to bother to have this projector fixed for this problem?
Let me know, please. Thanks.
The shutter is probably misaligned by a careless reassembly. Taking it back to where it was recently serviced is a good idea under the circumstances.
Thank you for asking your question here at JustAnswer.com.