"A large minority" of people in parts of Sandwell were not wearing face coverings in stores, despite them being mandatory, said Shaun Bailey, Conservative MP for West Bromwich West.
He called on ministers to give greater support to retail outlets to help them enforce restrictions.
Under current rules, people can be fined £100 for failing to wear a mask in a shop.
However, West Midlands Police have a policy of only using enforcement as a last resort, and many retailers allow people to walk around stores without wearing a face covering.
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Speaking during questions to Health Secretary Matt Hancock, Mr Bailey said: "I have been contacted by a large number of constituents over the weekend concerned about the fact that a large minority of people in retail settings are still not wearing face coverings.
"Most retailers are making sure that they follow the rules and they enforce them, but what support can the department give, across government and at local level, to ensure that when it comes to the small minority, we can enforce these rules and ensure, as the Secretary of State quite rightly said, that we are all in this together and we will stop the spread together?"
Mr Hancock said he believed more enforcement was "sadly necessary" to ensure that these rules are followed.
"If people follow the rules across the board, it will be easier to get a grip on the virus and the overall impact on the economy and on education, for instance, will be lower," he added.
"So that is the approach that we are taking both in the example he cites and more broadly."
Under new rules announced this week face masks are compulsory for restaurant and bar staff, and for customers who are not seated at a table.
Those who flout the rules can be fined £200.